This add-ons WAS once one of the best, meanwhile it is part of the dark side!
For details see here:
https://robertheaton.com/2018/07/02/stylish-browser-extension-steals-your-internet-history/

Das AddOn war mal eines der Besten, mittlerweile ist es Teil der dunklen Seite!
Mehr Details siehe Link oben (engl.!).

I knew that it was a kind of spyware, but I didn't even care about that, because it was a great addition and I was really wanted to use it. But moreover, it started consuming a lot of PC`s resources. For example, on only one tab with opened Google start page FF ate 20% of CPU and RAM leaked too much with no end. I investigated a little bit and figured out that was a Stylish's issue. After disabling Stylish everything was OK. Fortunately, there is another great alternative Stylus. It's not a spyware, no CPU/RAM consuming and works brilliantly. Thanks for Stylish, but I can't stand with this anymore, Stylus is my choice now.

Cant download at the site

Steals browser history

Great. The latest version breaks everything, and the developer doesn't even offer a version history so I could go back to an earlier version. Then I read the reviews and realize it became spyware at some point.

This is still spyware. Use either an OLD version from the original author (2.0.7 works fine) or Stylem created for Pale Moon and Basilisk, and based off Stylish 2.07, works fine on Fx 52.9 ESR and probably on other versions as well. Stylus is NOT a good choice since a main reason for using this extension is so one can modify the UI and you cannot do that with Stylus but you can with Stylem.

Found a way to reinstall itself and show a popup asking for permission to do data collection.

This addon is pure spyware now and should be removed from the store permanently. I don't know what Mozilla was thinking letting them back.

eu abaixei o firefox mais n da

Oh god, you brought it back? Single purpose extensions should NOT collect ANY user data.

Mozilla STOP indulge spyware like STEALish!

As said before. It collects your data in the really sneaky way. It was banned from AMO for some time.
Really sad that it returned here (guess because of updated description that now mentions that our data is collected).

Once a great extension, now spyware. Use "Stylus" instead.

I uninstalled this app when they started collecting all your information without disclosure and selling it, then they are finding ways to reinstall this addon without your permission. Please remove this addon from Firefox, it's on the level of malware now.

Who removes every single older version of their addon from addons.mozilla.org? A douche bag, that's who.

It is Spyware, steals user data and all history of visited URLs. More info: https://robertheaton.com/2018/07/02/stylish-browser-extension-steals-your-internet-history/

Use safe Stylus add-on instead!