Expand Theme Color meta tag docs (extend.md)

[coliff]

RE:

<meta name="theme-color" content="#ff69b4">

This is supported in Safari now too. We should review and revise these docs

REF: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color

[letscodedanish]

The theme-color value for the name attribute of the element indicates a suggested color that user agents should use to customize the display of the page or of the surrounding user interface. If specified, the content attribute must contain a valid CSS .