This is very exciting! I don't use bottles, because I barely game, but for rhose that do, I'm sure this is very exciting! (although some people would not like the Electron part, but hey, at least they are NOT abandoning GTK and will still offer it specifically for those users)
Generally the distro should handle that if you're not using something that requires manual intervention. If you are, Kvantum is pretty solid, it provides themes based on Adwaita or Libadwaita, as well as many other themes which KDE uses.
You'll have to set the theme to Kvantum which I think qt5ct and its Qt6 ilk should handle.
I used Heroic a few times and it's absolute garbage because it runs Electron.
They said they will offer these 2 versions side-by-side, but mark my words, a few months/years they will go with the Electron version and kill the GTK version.
Yeah, I'm guessing it's just a transition period, until most people use the new version, because it's easier, and then they will kill the old version because it "takes too much development effort" and "nobody uses it", and because it's a "duplication of effort".