Add polyfill comments to meta-snippet.html

[TimInTech]

Background: The parent issue notes that meta[name=theme-color] is not supported by Firefox, Firefox for Android, and Opera. To increase developer awareness and compatibility, add clear comments to meta-snippet.html documenting this lack of support and mentioning any available polyfills or workarounds.

Acceptance criteria:

• Add an explanatory comment near the meta[name=theme-color] entry in meta-snippet.html detailing browser support limitations.
• Reference or link to suitable polyfills or document the lack thereof.
• Ensure comments are clear for future maintainers.