theme-color
Fixes https://github.com/go-gitea/gitea/issues/24321. By not setting this meta tag, Safari will use body color for chrome and out-of-viewport areas, which looks much better then static mismatching green. As per [MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color) it's really only Apple browsers who still support this tag, most others have dropped support.