I mean, if you chose mozilla for privacy, don't choose this.

好！

Using way too much CPU, disabling it dropped firefox idle CPU usage from 20-30% to 1-3%. (Tested on Ubuntu and Arch Linux, on an Intel i7 6th gen laptop CPU)

This add-on is capable of doing a lot, and does everything it claims to do. My 3 star rating is because it is way beyond more confusing and/or complex then it needs to be, which causes tons of issues while browsing. As a new user with fresh eyes on this project I can offer this advice: 1) There should be an options setting that lets you disable styles by default on all sites (or enable by default). 2.) You should have the option to enable the styles for particular sites, even if they're generic "global" styles. 3.) It should be easy to whitelist sites that do not need the styles applied. With so many sites now offering dark themes, I do not need to apply unofficial themes over the top, increasing the likelihood of breaking the site. 4.) The devs should require informed intent acknowledgement to apply multiple styles at once, currently this happens by default and only results in the add-on seeming of poorer styling quality then it actually is, due to mismatched styles conflicting.

Solve these to a reasonable extent and I'll update my rating to what I know you deserve.