This extension isn't great, and it hasn't been updated since Feb 2019. But hopefully there's a solution to the dark theme not working in the Tree-Tabs sidebar, and hopefully there's a way to remove the horizontal tab bar?

The glaring white sidebar makes the dark theme pretty much useless, and also makes it hard to focus on items in the tree. Is there a way to make Tree-Tabs follow Opera's dark theme?

Also, how do I get rid of the duplicate/old horizontal tab bar? I don't need two sets of tabs. I can get rid of it in Firefox, how do I do it in Opera?