alx this might be related to the fact you can't override the CSS rules defined in an extension because the custom CSS is loaded after core but before extensions.
In your case you're probably trying to override CSS defined in the Tags extension, which is loaded after the custom CSS
The existing GitHub issue for that is here flarum/core#1726
You might be able to overcome it by using more a specific CSS selector, like
#app .TagLabel.colored .TagLabel-text. But maybe the
!important statements will mess with that.