XF 1.4 User banner customization in extra.css does not work


We have been trying to add custom user banners for different user groups via the extra.css file. I understand the fundamentals of adding classes like described here, but to no avail - nothing I ever add in extra.css (even with !important) will work. If I change the existing banners in user_banners.css, it works, but copying those classes to extra.css and changing them there won't work at all - everything I do there in regard to user banners is ignored. I wonder if there is some kind of override, because other customization works flawlessly - only anything done in regard to those banners is ignored completely. Does anyone have an idea why this wouldn't work?