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.