[3.0] Load the stylesheet a theme variant is recoloured with

[albertlast]

Description

A theme has two ways to recolour a variant, and Theme::loadVariant() only implements one of them.

The first is a single variants.css holding :root[data-variant="…"] blocks. That one is loaded.

The second is a whole stylesheet per variant sitting on top of index.css. The default theme documents it in index.template.php, right above the setting that declares the variants:

	/*
	 * Define the theme variants. Each variant has its own CSS file.
	 *
	 * Example:
	 * - index_red.css is loaded when the user selects the `red` variant.
	 *
	 * Additionally, a variants.css file is always loaded as well, in
	 * case you'd rather keep the styles in a single file or they're minimal.
	 */

and again further down:

			The most efficient way of writing multi themes is to use a master
			index.css plus variant.css files. If you've set them up properly
			(through Theme::$current->settings['theme_variants']), the variant files will be loaded
			for you automatically.

Nothing loaded them. This adds it.

Details

• order_pos is 2 — after index.css at 1 and beside dark.css, so the variant sheet overrides the base sheet, which is the point of it.
• The default variant is the base sheet on its own, so it has no file and is skipped.
• loadCSSFile() validates by default, so a theme that recolours through variants.css instead simply has nothing queued. Adding this cannot break such a theme.
• The default theme declares no variants, so nothing changes for it.

Verification

Declaring a sunset variant in the default theme locally, all three cases on a running forum with nothing added to log_errors:

case	result
index_sunset.css present, ?variant=sunset	loaded — its marker rule appears in the minified bundle
index_sunset.css absent, ?variant=sunset	nothing queued, page 200, nothing logged
index_default.css present, ?variant=default	correctly skipped

Issues References (Fixes|Related|Closes)

Related to #7933.

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