MacStache got it right, I was writing a long reply about Firefox but then I re-read the comment and remembered that I didn't even mention Firefox. ๐
However, I find that even if Firefox isn't a QT browser, it does a pretty good job of following colour schemes and only messes up on a handful of colour schemes. And turning on system titlebar works around the CSD so that's fine too. The one place it trips up on is following the icon pack, but that's just nitpicking at that point.
I know nothing of how Chromium looks/works these days, it's been like 6 years since I experimented with Vivaldi.
Yeah, it'd be nice to have a modern browser on QT which hopefully means I can try Plasma theming tricks like transparency and force blur on it. Technically this can be done on Firefox with CSS but system titlebar breaks that for some reason.
Even though you say that, there is a tutorial for the controls. Funny though, I forgot which button it is. ๐ง