Add PyQt theme presets and documented custom theme files

[ZFordDev]

Goal

Add a theme section to PyQt settings with built-in presets, including the dark appearance already shown in SchedPlus website material, plus a safe documented format for user-created themes.

Proposed presets

Use the established SnapDock palette families as design references where appropriate: Light, Dark, Solarized, Arctic Dark, and Forest. Adapt them to SchedPlus components rather than copying web CSS directly.

Scope

• Persist the selected theme in portable UI preferences.
• Apply theme changes predictably across all PyQt windows and dialogs.
• Support importing a documented theme file with schema/version validation.
• Define required semantic color tokens instead of accepting unrestricted executable or arbitrary application content.
• Consider an advanced color editor only after presets and theme-file import are stable.
• Keep Lite on its intentionally simple native presentation unless separately approved.

Acceptance criteria

• Built-in Light and Dark themes are release-ready; additional presets meet contrast standards.
• Every existing PyQt component is visually checked under each preset.
• Invalid theme files fail safely and leave the active theme unchanged.
• Backup and restore preserve portable theme preferences and custom theme data according to a documented policy.
• Public documentation explains every token and provides a complete example theme.
• Automated tests cover parsing, validation, persistence, and fallback behavior.