.background not limited to revaljs scope

[Mister-Hope]

Unlike other styles:

reveal.js/css/theme/template/theme.scss

Lines 327 to 331 in 1034983

	@media print {
	.backgrounds {
	background-color: var(--r-background-color);
	}
	}

The above do not limited to reveal.js scope, so it will affect users' .background class.