>>mig4ng+(OP)
Arc has native support for cosmetic improvements. Those are called Boosts and you can interactively create your owns in just a few clicks, and also add custom CSS/JS if needed.
I use this extensively to declutter/dedistract most websites I visit: Facebook, Twitter, Reddit, Medium, Gmail, YouTube, and so on. It's a real life-changer!
>>imbnwa+1E
Some would claim that is a feature. Though personally i'm trying hard(ish) to make Firefox' UI closer to early 2000s :-P (thanks to userChrome.css).