[SVG] SVG Favicon respects color-scheme of embedder

[hamishwillee]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
  or
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

Add a release note that SVG favicons with prefers-color-scheme will respect the color-scheme of the embedder.

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1764354

Other

prefers-color-scheme media query is supposed to reflect the user preference for a dark/light theme set via the device or browser settings. This came up because there was a bug report where an SVG favicon was not visible on firefox because the prefers-color-scheme was triggering on the OS (light theme) while firefox was set to dark theme.

The solution was to clarify that the prefers-color-scheme must follow the color-scheme of the embedder - i.e. a clarification of the spec.

This seems to fix the favicon case, but is being generalized in #19449.

[hamishwillee]

Status: Don

• BCD - not required
• Docs - not needed
• Release note FF101 Relnote: SVG images in UI respect color-scheme of embedder #19850
• dev-doc-complete

This is a very niche and focused change that will be generalized FF105: #19449. Agreed with developer worth just adding a release note for this particular part.