Changing "userChrome.css" Safe for Tor Browser?

I wanted to customize my Browsers UI by changing the userChrome but is this safe.
What should I take for precaution to understand what can intrude on my fingerprint and privacy?

I’m no expert on this, but I believe changing the Browsers UI in such a way should be safe. There are articles such as The Top Browser Fingerprinting Techniques Explained - Fingerprint which explain what sorts of things fingerprinting can use. I’d expect most purely UI elements shouldn’t be able to be used for fingerprinting.