Honestly, I don’t see why CSS theming is important. The customization is nice and all, but that’s not going to make people switch to Firefox. There are many other things that could be improved, like adding tab grouping. I use this extension called Tree Style Tab which I cannot live without. Firefox having something like that by default instead of an extension would be nice.
However, having said that, OperaGX did find quite a lot of success by simply making it easy to theme the browser, so I can see where they are coming from.
I would say this is an overall positive: The more people are aware of how easy it is to create fake this with AI the better, I would say. That would make people less likely to fall for it in the future, especially for something with greater consequences than just a parade that didn’t exist.