In the past I used a Firefox extension called Dust Me Selectors, but it appears to have been abandoned and isn't compatible with Firefox Quantum.
I've been searching for a while and I've failed to come up with a current alternative, other than http://unused-css.com which is a paid-for service. I'd rather have a tool I could use myself.
This must be something that every developer wonders at some point, so hopefully someone reading this will be able to point me in the right direction!
Sorry for the late answer, I've found your question when searching for the same thing. Anyway you might be interested by :
It removes unused css and you can add multiple urls.
Good luck
PS: I'm editing my answer because I found another website doing the unused CSS removal, maybe it will help someone reading this.