Don't set meta theme-color by default (#24340) (#24346)

Backport #24340 by @silverwind

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.
 
Before:
<img width="347" alt="Screenshot 2023-04-25 at 19 59 13"
src="https://user-images.githubusercontent.com/115237/234363180-0fe667ef-5469-4f5f-b31a-c9d73aff10ac.png">

After:
<img width="361" alt="Screenshot 2023-04-25 at 20 00 00"
src="https://user-images.githubusercontent.com/115237/234363185-e2d2b62f-0133-4316-944d-b614ffb84eb0.png">

Co-authored-by: silverwind <me@silverwind.io>

templates/base/head.tmpl

@@ -5,7 +5,7 @@
 	<meta name="viewport" content="width=device-width, initial-scale=1">
 	<title>{{if .Title}}{{.Title | RenderEmojiPlain}} - {{end}}{{if .Repository.Name}}{{.Repository.Name}} - {{end}}{{AppName}}</title>
 	{{if .ManifestData}}<link rel="manifest" href="data:{{.ManifestData}}">{{end}}
-	<meta name="theme-color" content="{{ThemeColorMetaTag}}">
+	{{if ThemeColorMetaTag}}<meta name="theme-color" content="{{ThemeColorMetaTag}}">{{end}}
 	<meta name="default-theme" content="{{DefaultTheme}}">
 	<meta name="author" content="{{if .Repository}}{{.Owner.Name}}{{else}}{{MetaAuthor}}{{end}}">
 	<meta name="description" content="{{if .Repository}}{{.Repository.Name}}{{if .Repository.Description}} - {{.Repository.Description}}{{end}}{{else}}{{MetaDescription}}{{end}}">