There’s only the old Breeze theme.
I also found out that it’s impossible to search for GTK styles.
havent seen a Union port for gtk. breeze-gtk is still the usual path for gtk3 apps.
gtk4 theming mostly doesnt follow that the same way.
id try:
-install breeze-gtk (or gtk3-breeze-theme, name depends on distro)
-Settings → Colors & Themes → Application Style → Configure GNOME/GTK Application Style → Breeze
-flatpaks: flatpak override --user --env=GTK_THEME=Breeze
yeah that search bug for gtk styles looks right.
That is most likely a matter of priorities.
Primary targets for Union are QtWidgets and QtQuick Controls, other toolkits/component sets will have to come after these have been stabilized.
Not yet. A GTK output backend is possible… if people build it. 
It remains to be seen how applicable it would be though, in a world where an increasing number of GTK apps use Libadwaita and therefore don’t want to be themed.
You can set themes for libadwaita too, with GTK_THEME (as mentioned), but that mostly goes against the developers and you take responsibility.
There is also my library, if you’d like to unify the title bar of these apps (to all use the system title bar/SSD)
This is even further unsupported by either GTK or libadwaita or the app developers, but if you have issues with it, you can write one and I’d check it.
I don’t have many libadwaita apps, but how does this handle apps that have the settings menu integrated into the title bar?
Does it remove it completely as well? Or does it move it into a row-style menu?
If it does remove it, does that mean the user can only access settings via shortcuts?
Oh, wait. I just saw that there are some examples far down the page. I see.
