diff --git a/.tool-versions b/.tool-versions
index fa6bee2..d73a169 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -5,3 +5,4 @@ uv 0.4.18
 copilot-cli 1.0.69
 squad-cli 0.10.0
 gh 2.92.0
+delta 0.19.2
diff --git a/scripts/linux/tools/delta.sh b/scripts/linux/tools/delta.sh
new file mode 100644
index 0000000..5bbe84d
--- /dev/null
+++ b/scripts/linux/tools/delta.sh
@@ -0,0 +1,111 @@
+#!/usr/bin/env bash
+# scripts/linux/tools/delta.sh -- Install git-delta (syntax-highlighting git pager) at pinned version
+#
+# Called by: scripts/linux/setup.sh
+# Idempotent: yes -- version-aware; upgrades if installed version != pinned version.
+# Opt-in: NOT in DEFAULT_TOOLS; only runs when requested via --only=delta.
+#
+# Ubuntu 22.04+: installs via apt-get (git-delta package).
+# Older/non-apt Linux: downloads the pinned release tarball from GitHub releases.
+# macOS: brew install git-delta (warns if brew version differs from pin).
+#
+# Defines apply_delta_git_config() for testability.
+# Main install block is guarded by BASH_SOURCE/argv0 check so this file can
+# be sourced in tests to call apply_delta_git_config in isolation.
+
+# shellcheck disable=SC1091
+. "$(dirname "${BASH_SOURCE[0]}")/../lib/log.sh"
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+DELTA_VERSION="$(sh "${SCRIPT_DIR}/../../lib/read-tool-version.sh" delta)"
+
+apply_delta_git_config() {
+  log_info "Applying global git config for delta..."
+  git config --global core.pager delta
+  git config --global interactive.diffFilter 'delta --color-only'
+  git config --global delta.navigate true
+  git config --global delta.dark true
+  # Light-mode override: git config --global delta.dark false
+  git config --global merge.conflictStyle zdiff3
+  log_ok "delta git config applied (core.pager=delta, dark=true)"
+}
+
+if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+  set -euo pipefail
+
+  # Detect installed version
+  INSTALLED_VERSION=""
+  if command -v delta &>/dev/null; then
+    INSTALLED_VERSION="$(delta --version 2>&1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+' | head -1 || true)"
+  fi
+
+  if [ "${INSTALLED_VERSION}" = "${DELTA_VERSION}" ]; then
+    log_ok "git-delta already at pinned version ${DELTA_VERSION}"
+    apply_delta_git_config
+    exit 0
+  fi
+
+  if [ -n "${INSTALLED_VERSION}" ]; then
+    log_info "git-delta ${INSTALLED_VERSION} installed; upgrading to pinned ${DELTA_VERSION}..."
+  else
+    log_info "Installing git-delta ${DELTA_VERSION}..."
+  fi
+
+  PLATFORM="$(uname -s)"
+  if [[ "$PLATFORM" == "Darwin" ]]; then
+    # Homebrew: versioned formulae for git-delta are not reliably pinnable.
+    if command -v delta &>/dev/null; then
+      brew upgrade git-delta || true
+    else
+      brew install git-delta
+    fi
+    ACTUAL_VERSION="$(delta --version 2>&1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+' | head -1 || echo 'unknown')"
+    if [ "${ACTUAL_VERSION}" != "${DELTA_VERSION}" ]; then
+      log_warn "git-delta ${ACTUAL_VERSION} installed (pinned: ${DELTA_VERSION}); brew cannot guarantee exact version"
+    else
+      log_ok "git-delta installed at ${DELTA_VERSION}"
+    fi
+  else
+    # Linux: prefer apt-get (Ubuntu 22.04+), fall back to pinned tarball
+    APT_OK=0
+    if command -v apt-get &>/dev/null; then
+      log_info "Trying apt-get install git-delta..."
+      sudo apt-get install -y git-delta 2>/dev/null && APT_OK=1 || APT_OK=0
+    fi
+
+    if [ "$APT_OK" -eq 1 ]; then
+      ACTUAL_VERSION="$(delta --version 2>&1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+' | head -1 || echo 'unknown')"
+      if [ "${ACTUAL_VERSION}" != "${DELTA_VERSION}" ]; then
+        log_warn "apt installed git-delta ${ACTUAL_VERSION} (pinned: ${DELTA_VERSION}); use tarball for exact pin"
+      fi
+      log_ok "git-delta installed via apt at ${ACTUAL_VERSION}"
+    else
+      # Tarball: asset name delta-<version>-<arch-triple>.tar.gz
+      ARCH="$(uname -m)"
+      case "$ARCH" in
+        x86_64)        ARCH_TRIPLE="x86_64-unknown-linux-gnu" ;;
+        aarch64|arm64) ARCH_TRIPLE="aarch64-unknown-linux-gnu" ;;
+        *)             log_error "Unsupported architecture: ${ARCH}"; exit 1 ;;
+      esac
+
+      TARBALL="delta-${DELTA_VERSION}-${ARCH_TRIPLE}.tar.gz"
+      TARBALL_URL="https://github.com/dandavison/delta/releases/download/${DELTA_VERSION}/${TARBALL}"
+      INSTALL_DIR="${HOME}/.local/bin"
+      WORK_DIR="${HOME}/.local/share/dev-setup-install/delta"
+
+      mkdir -p "$INSTALL_DIR"
+      mkdir -p "$WORK_DIR"
+
+      log_info "Downloading ${TARBALL}..."
+      curl -fsSL "$TARBALL_URL" -o "${WORK_DIR}/${TARBALL}"
+      tar -xzf "${WORK_DIR}/${TARBALL}" -C "$WORK_DIR"
+      cp "${WORK_DIR}/delta-${DELTA_VERSION}-${ARCH_TRIPLE}/delta" "${INSTALL_DIR}/delta"
+      chmod +x "${INSTALL_DIR}/delta"
+      rm -rf "$WORK_DIR"
+
+      log_ok "git-delta ${DELTA_VERSION} installed to ${INSTALL_DIR}/delta"
+    fi
+  fi
+
+  apply_delta_git_config
+fi
diff --git a/scripts/windows/setup.ps1 b/scripts/windows/setup.ps1
index 393c5e5..c5bac3d 100644
--- a/scripts/windows/setup.ps1
+++ b/scripts/windows/setup.ps1
@@ -46,6 +46,7 @@ $ErrorActionPreference = 'Stop'
 . "$PSScriptRoot\tools\profile.ps1"
 . "$PSScriptRoot\tools\auth.ps1"
 . "$PSScriptRoot\tools\git-hook.ps1"
+. "$PSScriptRoot\tools\delta.ps1"
 
 # ---------------------------------------------------------------------------
 # $DefaultTools -- single ordered source of truth for a no-arg default run.
@@ -85,6 +86,7 @@ $ToolRegistry = [ordered]@{
     'dotfiles'     = { Install-Dotfiles }
     'profile'      = { Write-PowerShellProfile }
     'git-hook'     = { Install-GitHook }
+    'delta'        = { Install-Delta }
 }
 
 # ---------------------------------------------------------------------------
diff --git a/scripts/windows/tools/delta.ps1 b/scripts/windows/tools/delta.ps1
new file mode 100644
index 0000000..71cb7ea
--- /dev/null
+++ b/scripts/windows/tools/delta.ps1
@@ -0,0 +1,73 @@
+# scripts/windows/tools/delta.ps1 - git-delta installer
+#
+# Installs git-delta at pinned version from .tool-versions.
+# Opt-in: NOT in $DefaultTools; only runs when requested via -Only 'delta'.
+# winget preferred (id dandavison.delta); scoop fallback when winget unavailable.
+#
+# Defines Set-DeltaGitConfig for testability: applies global git config for
+# delta without requiring a full install run.
+# PS 5.1 ASCII-only: no smart quotes, em-dashes, or non-ASCII characters.
+
+Set-StrictMode -Version Latest
+$ErrorActionPreference = 'Stop'
+
+. "$PSScriptRoot\..\lib\logging.ps1"
+. "$PSScriptRoot\..\lib\path.ps1"
+. "$PSScriptRoot\..\..\lib\Read-ToolVersion.ps1"
+
+function Set-DeltaGitConfig {
+    [CmdletBinding(SupportsShouldProcess)]
+    param()
+    if (-not $PSCmdlet.ShouldProcess('global git config', 'Set delta pager settings')) { return }
+    Write-Info "Applying global git config for delta..."
+    git config --global core.pager delta
+    git config --global interactive.diffFilter 'delta --color-only'
+    git config --global delta.navigate true
+    git config --global delta.dark true
+    # Light-mode override: git config --global delta.dark false
+    git config --global merge.conflictStyle zdiff3
+    Write-Ok "delta git config applied (core.pager=delta, dark=true)"
+}
+
+function Install-Delta {
+    $DeltaVersion = Get-ToolVersion -Name 'delta'
+
+    # Detect installed version
+    $InstalledVersion = ''
+    if (Get-Command delta -ErrorAction SilentlyContinue) {
+        $raw = (delta --version 2>&1) | Select-Object -First 1 | Out-String
+        $m = [regex]::Match($raw, '[0-9]+\.[0-9]+\.[0-9]+')
+        if ($m.Success) { $InstalledVersion = $m.Value }
+    }
+
+    if ($InstalledVersion -eq $DeltaVersion) {
+        Write-Ok "git-delta already at pinned version $DeltaVersion"
+        Set-DeltaGitConfig
+        return
+    }
+
+    if ($InstalledVersion) {
+        Write-Info "git-delta $InstalledVersion installed; upgrading to pinned $DeltaVersion..."
+    } else {
+        Write-Info "Installing git-delta $DeltaVersion..."
+    }
+
+    # winget preferred; fall back to scoop if winget is unavailable
+    if (Get-Command winget -ErrorAction SilentlyContinue) {
+        winget install --id dandavison.delta --version $DeltaVersion --silent `
+            --accept-source-agreements --accept-package-agreements
+        Assert-LastExit -ToolName "git-delta" -AllowedExitCodes @(0, -1978335189)
+        Refresh-SessionPath
+        Write-Ok "git-delta installed via winget at $DeltaVersion"
+    } elseif (Get-Command scoop -ErrorAction SilentlyContinue) {
+        Write-Info "winget not available; falling back to scoop..."
+        scoop install delta
+        Assert-LastExit -ToolName "git-delta (scoop)"
+        Write-Warn "scoop installed latest delta; version may differ from pinned $DeltaVersion"
+    } else {
+        Write-Err "Neither winget nor scoop available; cannot install git-delta"
+        throw "git-delta install failed: no supported package manager found"
+    }
+
+    Set-DeltaGitConfig
+}
diff --git a/tests/test_delta_installer.sh b/tests/test_delta_installer.sh
new file mode 100644
index 0000000..2e5cf24
--- /dev/null
+++ b/tests/test_delta_installer.sh
@@ -0,0 +1,157 @@
+#!/usr/bin/env bash
+# tests/test_delta_installer.sh -- parity tests for git-delta opt-in installer (#466)
+#
+# Tests:
+#   T_delta_in_list        -- delta appears in --list output (opt-in discoverable)
+#   T_delta_not_default    -- delta is NOT installed by a default no-arg run
+#   T_delta_gitconfig_iso  -- apply_delta_git_config writes core.pager=delta under
+#                             an isolated GIT_CONFIG_GLOBAL (does not touch ~/.gitconfig)
+#   T_delta_gitconfig_idem -- apply_delta_git_config is idempotent (safe to run twice)
+#
+# Usage: bash tests/test_delta_installer.sh
+# Requires: bash 3.2+ (macOS compatible), git
+
+set -uo pipefail
+
+PASS=0
+FAIL=0
+SKIP=0
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[0;33m'
+RESET='\033[0m'
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"
+LINUX_SETUP="${REPO_ROOT}/scripts/linux/setup.sh"
+DELTA_SH="${REPO_ROOT}/scripts/linux/tools/delta.sh"
+STUB_DIR="${REPO_ROOT}/tests/fixtures/stub-tools/linux"
+
+pass() { printf "${GREEN}PASS${RESET}: %s\n" "$1"; PASS=$((PASS + 1)); }
+fail() { printf "${RED}FAIL${RESET}: %s\n" "$1"; FAIL=$((FAIL + 1)); }
+# shellcheck disable=SC2329
+skip() { printf "${YELLOW}SKIP${RESET}: %s -- %s\n" "$1" "$2"; SKIP=$((SKIP + 1)); }
+
+# Log no-ops -- inherited by subshells; called indirectly by delta.sh when sourced.
+# shellcheck disable=SC2329
+log_info() { :; }
+# shellcheck disable=SC2329
+log_ok()   { :; }
+# shellcheck disable=SC2329
+log_warn() { :; }
+# shellcheck disable=SC2329
+log_error(){ :; }
+
+# ---------------------------------------------------------------------------
+# T_delta_in_list: --list on real tools dir includes 'delta'
+# Fails RED when scripts/linux/tools/delta.sh does not exist.
+# ---------------------------------------------------------------------------
+echo ""
+echo "--- T_delta_in_list ---"
+list_out="$(bash "$LINUX_SETUP" --list 2>&1)" && list_exit=$? || list_exit=$?
+if [ "$list_exit" -ne 0 ]; then
+  fail "T_delta_in_list: --list exited $list_exit (expected 0)"
+elif echo "$list_out" | grep -qF "delta"; then
+  pass "T_delta_in_list: delta appears in --list output (opt-in discoverable)"
+else
+  fail "T_delta_in_list: delta missing from --list output (is delta.sh in tools/?)"
+  echo "  --list output: $list_out"
+fi
+
+# ---------------------------------------------------------------------------
+# T_delta_not_default: default no-arg run with stub dir does NOT run delta
+# Delta is opt-in; it must not appear in defaults.txt.
+# ---------------------------------------------------------------------------
+echo ""
+echo "--- T_delta_not_default ---"
+RUN_LOG="$(mktemp)"
+export RUN_LOG
+bash "$LINUX_SETUP" "--tools-dir=${STUB_DIR}" >/dev/null 2>&1 || true
+if grep -qF "delta" "$RUN_LOG" 2>/dev/null; then
+  fail "T_delta_not_default: delta ran in a default no-arg install (must be opt-in only)"
+else
+  pass "T_delta_not_default: delta does NOT run in a default install (correctly opt-in)"
+fi
+rm -f "$RUN_LOG"
+unset RUN_LOG
+
+# ---------------------------------------------------------------------------
+# T_delta_gitconfig_iso: apply_delta_git_config writes core.pager=delta under
+# an isolated GIT_CONFIG_GLOBAL; real ~/.gitconfig is never touched.
+# Fails RED when delta.sh does not exist (function not defined).
+# ---------------------------------------------------------------------------
+echo ""
+echo "--- T_delta_gitconfig_iso ---"
+if [ ! -f "$DELTA_SH" ]; then
+  fail "T_delta_gitconfig_iso: delta.sh not found at $DELTA_SH -- RED (pre-implementation)"
+else
+  ISO_CFG="$(mktemp)"
+  # Run in a subshell so set -euo pipefail from delta.sh does not affect parent.
+  # GIT_CONFIG_GLOBAL is set inside the subshell intentionally; we read via inline
+  # env assignment after the subshell exits.
+  # shellcheck disable=SC2030
+  (
+    export GIT_CONFIG_GLOBAL="$ISO_CFG"
+    # shellcheck disable=SC1090
+    . "$DELTA_SH"
+    apply_delta_git_config
+  )
+  sub_exit=$?
+  if [ "$sub_exit" -ne 0 ]; then
+    fail "T_delta_gitconfig_iso: apply_delta_git_config subshell exited $sub_exit"
+  else
+    # shellcheck disable=SC2031
+    got_pager="$(GIT_CONFIG_GLOBAL="$ISO_CFG" git config --global --get core.pager 2>/dev/null || true)"
+    # shellcheck disable=SC2031
+    got_filter="$(GIT_CONFIG_GLOBAL="$ISO_CFG" git config --global --get interactive.diffFilter 2>/dev/null || true)"
+    if [ "$got_pager" = "delta" ] && [ "$got_filter" = "delta --color-only" ]; then
+      pass "T_delta_gitconfig_iso: core.pager=delta and interactive.diffFilter set under isolated config"
+    else
+      fail "T_delta_gitconfig_iso: unexpected config values (pager='${got_pager}', filter='${got_filter}')"
+    fi
+  fi
+  rm -f "$ISO_CFG"
+fi
+
+# ---------------------------------------------------------------------------
+# T_delta_gitconfig_idem: running apply_delta_git_config twice does not error
+# (git config --global is naturally idempotent).
+# Fails RED when delta.sh does not exist.
+# ---------------------------------------------------------------------------
+echo ""
+echo "--- T_delta_gitconfig_idem ---"
+if [ ! -f "$DELTA_SH" ]; then
+  fail "T_delta_gitconfig_idem: delta.sh not found -- RED (pre-implementation)"
+else
+  ISO_CFG2="$(mktemp)"
+  # shellcheck disable=SC2030,SC2031
+  (
+    # shellcheck disable=SC2031
+    export GIT_CONFIG_GLOBAL="$ISO_CFG2"
+    # shellcheck disable=SC1090
+    . "$DELTA_SH"
+    apply_delta_git_config  # first run
+    apply_delta_git_config  # second run -- must be a no-op / safe
+  )
+  idem_exit=$?
+  if [ "$idem_exit" -ne 0 ]; then
+    fail "T_delta_gitconfig_idem: second run of apply_delta_git_config exited $idem_exit"
+  else
+    # shellcheck disable=SC2031
+    got_dark="$(GIT_CONFIG_GLOBAL="$ISO_CFG2" git config --global --get delta.dark 2>/dev/null || true)"
+    if [ "$got_dark" = "true" ]; then
+      pass "T_delta_gitconfig_idem: idempotent double-run completed; delta.dark=true"
+    else
+      fail "T_delta_gitconfig_idem: delta.dark unexpected value after double-run: '${got_dark}'"
+    fi
+  fi
+  rm -f "$ISO_CFG2"
+fi
+
+# ---------------------------------------------------------------------------
+echo ""
+echo "Results: ${PASS} passed, ${FAIL} failed, ${SKIP} skipped"
+if [ "$FAIL" -gt 0 ]; then
+  exit 1
+fi
+exit 0
diff --git a/tests/test_delta_installer_pwsh.ps1 b/tests/test_delta_installer_pwsh.ps1
new file mode 100644
index 0000000..7607de7
--- /dev/null
+++ b/tests/test_delta_installer_pwsh.ps1
@@ -0,0 +1,143 @@
+# tests/test_delta_installer_pwsh.ps1 -- parity tests for git-delta opt-in installer (#466)
+#
+# Tests:
+#   T_delta_in_list       -- delta appears in -List output (opt-in discoverable)
+#   T_delta_not_default   -- delta is NOT installed by a default no-arg run
+#   T_delta_gitconfig_iso -- Set-DeltaGitConfig writes core.pager=delta under an
+#                            isolated GIT_CONFIG_GLOBAL (does not touch real config)
+#   T_delta_gitconfig_idem -- Set-DeltaGitConfig is idempotent (safe to run twice)
+#
+# Usage: powershell -ExecutionPolicy Bypass -File tests\test_delta_installer_pwsh.ps1
+# PS 5.1 ASCII-only: no smart quotes, em-dashes, or non-ASCII characters.
+
+$ErrorActionPreference = 'Stop'
+$TestsPassed  = 0
+$TestsFailed  = 0
+$TestsSkipped = 0
+
+$RepoRoot   = Split-Path $PSScriptRoot -Parent
+$WinSetup   = Join-Path $RepoRoot 'scripts\windows\setup.ps1'
+$DeltaPs1   = Join-Path $RepoRoot 'scripts\windows\tools\delta.ps1'
+$StubDir    = Join-Path $RepoRoot 'tests\fixtures\stub-tools\windows'
+
+function Test-Scenario {
+    param([string]$Name, [scriptblock]$Test)
+    Write-Host "`n=== TEST: $Name ===" -ForegroundColor Cyan
+    try {
+        & $Test
+        Write-Host "[PASS] $Name" -ForegroundColor Green
+        $script:TestsPassed++
+    }
+    catch {
+        Write-Host "[FAIL] $Name" -ForegroundColor Red
+        Write-Host "  Error: $_" -ForegroundColor Red
+        $script:TestsFailed++
+    }
+}
+
+function Write-Skip {
+    param([string]$Name, [string]$Reason)
+    Write-Host "`n=== TEST: $Name ===" -ForegroundColor Cyan
+    Write-Host "[SKIP] $Name -- $Reason" -ForegroundColor Yellow
+    $script:TestsSkipped++
+}
+
+# ---------------------------------------------------------------------------
+# T_delta_in_list: -List on real registry includes 'delta'
+# Fails RED when delta.ps1 is not dot-sourced + registered in setup.ps1.
+# ---------------------------------------------------------------------------
+
+Test-Scenario "T_delta_in_list: delta appears in -List output (opt-in discoverable)" {
+    $out = powershell -NoProfile -ExecutionPolicy Bypass -File $WinSetup -List 2>&1 | Out-String
+    $ec  = $LASTEXITCODE
+    if ($ec -ne 0) { throw "-List exited $ec (expected 0)" }
+    if ($out -notmatch '\bdelta\b') {
+        throw "delta missing from -List output; has delta.ps1 been registered in ToolRegistry?"
+    }
+}
+
+# ---------------------------------------------------------------------------
+# T_delta_not_default: default no-arg run with stub dir does NOT run delta
+# ---------------------------------------------------------------------------
+
+Test-Scenario "T_delta_not_default: delta does NOT run in a default install (opt-in only)" {
+    $runLog = [System.IO.Path]::GetTempFileName()
+    $env:RUN_LOG = $runLog
+    try {
+        powershell -NoProfile -ExecutionPolicy Bypass -File $WinSetup `
+            -ToolsDir $StubDir 2>&1 | Out-Null
+        $content = Get-Content $runLog -ErrorAction SilentlyContinue
+        if ($content -contains 'delta') {
+            throw "delta ran in a default no-arg install (must be opt-in only)"
+        }
+    }
+    finally {
+        $env:RUN_LOG = $null
+        Remove-Item $runLog -ErrorAction SilentlyContinue
+    }
+}
+
+# ---------------------------------------------------------------------------
+# T_delta_gitconfig_iso: Set-DeltaGitConfig writes core.pager=delta under
+# an isolated GIT_CONFIG_GLOBAL; real ~/.gitconfig is never touched.
+# Fails RED when delta.ps1 does not exist.
+# ---------------------------------------------------------------------------
+
+Test-Scenario "T_delta_gitconfig_iso: core.pager=delta written to isolated config" {
+    if (-not (Test-Path $DeltaPs1)) {
+        throw "delta.ps1 not found at $DeltaPs1 -- RED (pre-implementation)"
+    }
+    $isolatedConfig = [System.IO.Path]::GetTempFileName()
+    $savedConfig = $env:GIT_CONFIG_GLOBAL
+    try {
+        $env:GIT_CONFIG_GLOBAL = $isolatedConfig
+        # Dot-source delta.ps1 to load Set-DeltaGitConfig (and Install-Delta)
+        # Install-Delta is defined but not called here.
+        . $DeltaPs1
+        Set-DeltaGitConfig
+        $pager = git config --global --get core.pager
+        if ($pager -ne 'delta') {
+            throw "core.pager expected 'delta', got '$pager'"
+        }
+        $filter = git config --global --get interactive.diffFilter
+        if ($filter -ne 'delta --color-only') {
+            throw "interactive.diffFilter expected 'delta --color-only', got '$filter'"
+        }
+    }
+    finally {
+        $env:GIT_CONFIG_GLOBAL = $savedConfig
+        Remove-Item $isolatedConfig -ErrorAction SilentlyContinue
+    }
+}
+
+# ---------------------------------------------------------------------------
+# T_delta_gitconfig_idem: running Set-DeltaGitConfig twice does not error
+# ---------------------------------------------------------------------------
+
+Test-Scenario "T_delta_gitconfig_idem: Set-DeltaGitConfig is idempotent (safe to run twice)" {
+    if (-not (Test-Path $DeltaPs1)) {
+        throw "delta.ps1 not found at $DeltaPs1 -- RED (pre-implementation)"
+    }
+    $isolatedConfig = [System.IO.Path]::GetTempFileName()
+    $savedConfig = $env:GIT_CONFIG_GLOBAL
+    try {
+        $env:GIT_CONFIG_GLOBAL = $isolatedConfig
+        . $DeltaPs1
+        Set-DeltaGitConfig  # first run
+        Set-DeltaGitConfig  # second run -- must not error
+        $dark = git config --global --get delta.dark
+        if ($dark -ne 'true') {
+            throw "delta.dark expected 'true' after double-run, got '$dark'"
+        }
+    }
+    finally {
+        $env:GIT_CONFIG_GLOBAL = $savedConfig
+        Remove-Item $isolatedConfig -ErrorAction SilentlyContinue
+    }
+}
+
+# ---------------------------------------------------------------------------
+Write-Host ""
+Write-Host "Results: $TestsPassed passed, $TestsFailed failed, $TestsSkipped skipped"
+if ($TestsFailed -gt 0) { exit 1 }
+exit 0