Spyware.

UPDATE: Do NOT use this extension. After some research into this application and monitoring the traffic it passes, I have come to the conclusion that this app is essentially spyware. It logs an inordinate unneeded amount of personal and identifiable information on your browsing habits and then sends them back to the parent company who based on investigations, is likely selling that data to any willing 3rd party. More so, the companies security of this data is largely in question. So even if they werent selling it, they arent doing much to protect it and as a result, its only a matter of time until its stolen, or we are made aware that its been stolen.

This extension is nice, but it will randomly just lose all of its settings. I was using the app for a few days in a very controlled development environment and was shocked to find that randomly one day my custom styles were completely gone and the app acted as though i was using a brand new fresh install of the add on. To make matters more insulting, when i reached out to support about it, their answer was entirely unhelpful and their response was along the lines of "it wasn't our fault, we cant help you, not our problem". This is most rude and unfortunately and as a result I will not be using this product any more and would recommend if you have any requirement for even the slightest bit of reliability that you also forgo the use of this horribly made, poorly supported application.

Having used Stylish for years, I was very surprised to see that the new version casually asks if you'd like to send *all* of your browsing information to Stylish. Duh. No. But then, when I went to simply use Stylish, the menu now shows a big picture of a crying kitten, yes, literally, telling me I didn't enable it correctly and asking me to please, please do it. What the heck!? Why am I being badgered to give up my privacy? This is just weird.

[UPDATE: A quick Google shows me that some company *bought* Stylish and is trying to monetize it by spying on everyone who had trusted the previous owners. This is despicable. Mozilla ought not allow ownership of an add-on to be transfered without a big warning on the add-on and automatic updates should be disabled. I'm switching to Stylus which respects my privacy: it collects no data about me because there's no reason for a theme add-on like this to do so.]

Now contains spyware.
Get Stylus instead

Now contains spyware.
Get Stylus instead.

Now contains spyware.

Now contains spyware.
Get Stylus instead.

Now contains spyware.
Get Stylus instead.

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.