UI: Use static function instead of macro to set color

Macros can be hard to read and are usually not very friendly to use in a
debugger. Using a static function instead gives the advantage of better
syntax highlighting in IDEs and better debugger support.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f