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

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.

modules/setting/ui.go

@@ -77,7 +77,7 @@ var UI = struct {
 	GraphMaxCommitNum:   100,
 	CodeCommentLines:    4,
 	ReactionMaxUserNum:  10,
-	ThemeColorMetaTag:   `#6cc644`,
+	ThemeColorMetaTag:   ``,
 	MaxDisplayFileSize:  8388608,
 	DefaultTheme:        `auto`,
 	Themes:              []string{`auto`, `gitea`, `arc-green`},