Hmm, not a lot to go off of then.
Can you just go through these steps to see if anything isn't set up correctly?
https://www.userchrome.org/how-create-userchrome-css.html
You can also check via the Browser Toolbox, whether the file gets loaded.
In the "Style Editor" tab, you can filter the list on the left and if "userChrome.css" isn't listed there, then it didn't get loaded.
If it is listed in the Style Editor, then something about your rules is broken. You might be missing !important in places.
Or your CSS rules might not target the right classes/ids/variables anymore. For example, I had a breakage about a month ago, because they renamed the CSS variable --toolbarbutton-inner-padding to --toolbarbutton-padding-inner.

Ah, guter Punkt. Soweit um die Ecke hatte ich nicht gedacht.