I know you said that you applied some CSS but have you applied "username CSS" per user group like it describes
here? If you have applied a colour property to that and it fails to show up in some parts of the site, it might be that something is overriding it. Try applying an !important rule to the colour property like it shows
here.
I'm not a XenForo expert but from some brief research, username CSS should apparently apply site wide.
Hopefully that's of help. If not, I can try to help in a different way. You can try applying the style and I can try to poke around in Firefox's dev tools to find out why it's styled differently on different parts of the forum. XenForo just renders regular CSS at the end of the day.