Steps to replicate the issue (include links if applicable):

1. Go to https://en.wikipedia.org/wiki/User_talk:Martadoulia?safemode=1&useskin=vector-2022&vectornightmode=1 for an example in dark mode.
2. The SockBlock template from Template:SockBlock is very difficult to read (note the template has a workaround now so the current version won't reproduce the bug).

For a more minimal reproduction, try this example on any English Wikipedia page with dark mode enabled:

{{#invoke:Message box|tmbox|style=background:var(--background-color-warning-subtle);|text=The quick brown fox jumps over the lazy dog.}}

What happens?:

Any element with a style attribute that includes a background value but no explicit color gets a hard-coded light-mode foreground color (#202122).

This comes from here:

https://github.com/wikimedia/mediawiki-extensions-WikimediaMessages/blame/7f938a14f5f639092b2453a51ec038edb601b078/modules/ext.wikimediamessages.styles/theme-night.less#L87

// T358797 - if a background color is specified, assume they wanted the day mode font color
.mw-parser-output [ style*='background' ] {
	/* @color-base */
	color: #202122; // not !important so that if a color is also specified it will take priority
}

This makes text unreadable in dark mode when templates or modules set a background using Codex variables.

What should have happened instead?:

The override should not apply when the background color uses a Codex variable like var(--background-color-warning-subtle). It should also use a more neutral fallback color to minimize the chance of the text being unreadable.

Proposed fix:

.mw-parser-output [style*='background']:not([style*='--background-color']) {
	/* @color-base */
	color: #667755; /* not !important so that if a color is also specified it will take priority */
}

1. This preserves the intended workaround for inline hard-coded backgrounds but avoids breaking Codex-based ones.
2. It also uses a fallback color that is readable against most backgrounds in both light and dark modes. While #808080 is a medium gray and would be a reasonable fallback color, #667755 will provide better contrast on more Codex backgrounds. That's based on exploring the color space against each Codex light and dark background (aiming for a minimum 3:1 contrast ratio, which is not ideal, but this is a fallback that is straining to work for both light and dark backgrounds).

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

N/A (English Wikipedia)

Other information (browser name/version, screenshots, etc.):

Tested in Firefox 144.