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And should be optional. Yes, it's true that the old popup UI needed improvements. But it didn't need to be replaced by the current monstrosity that is this new UI.
Here is a list of some of the issues it has:
It wastes screen space.
The popup is gigantic now. This wouldn't normally be a problem, because large popups mean more useful features, right? Nope, instead you enlarged it just so you could add pointless things like USELESS GIGANTIC THUMBNAILS, install stats, author names, etc. None of that is relevant when all I'm trying to do is toggle/edit styles. Also, the font sizes, element spacing, and element sizes are so large that it looks like a mobile app. Complete waste of space.
Since the USELESS GIGANTIC THUMBNAILS are so large, you have to scroll down to see more than 2 styles. And to add insult to injury, there is no indication that scrolling is needed, because in your quest to adhere by the latest shitty fake-minimalist design fad, you left out the inclusion of a scrollbar.
It wastes resources.
So now I have to open the options page almost every time. This means the UI is worse than bad: it's downright useless. You made a poorly-designed UI and didn't make sure its most vital function actually worked.
Seriously, who designed this new UI? It's such a departure from your previously-good design and functionality principles that it makes me wonder if you brought in some clueless third party to make it (edit: oh wait, you did). With the inclusion of this bad UI, you've made the same mistake that a lot of other developers have been making recently: changing a UI just for the sake of change, rather than to actually improve it. User interface design is more than about just making things pretty, it's about making it actually functional and usable.
I've been using Stylish for about 10 years and have depend heavily on it to tame the web. I've made many private userstyles, and a few public ones. I'm one of its most loyal fans. Stylish has been a household name for almost 15 years now. It has single-handedly created the userstyle scene. Yet this new UI is so bad that, coupled with the new user-info-selling practices, I'm considering ditching Stylish and finding a better alternative.
Many other people have recently given you similar feedback about the new UI, yet zero response from you. Honestly, if this UI isn't fixed/removed, Stylish for Chrome is officially dead to me. I'm begging you to fix this.