privacy.resistFingerprinting breaks a lot more than just themes. Many of the weird problems reported in Firefox (and forks) are just from enabling it.

It has some pros but also TONNES of cons. Everything from a completely blank page to wrong timestamps to poor textures and so much more. Sometimes you will be flagged as a bot and prompted with literally infinite puzzles, thus effectively banning you from a website.

Some of these problems get fixed but new ones also get born. I personally use it but I also expect breakage and worse performance.