OS: Bullseye (stable) QT-Curve was never ported to GTK3 and because of this, we struggle to have apps like GIMP to use our QT-Curve-GTK2 theme. If we choose a GTK Theme from the new GTK-Application Style KCM then it will choose a uniform look for GTK3/GTK2 apps. I'm currently using "gtk-chtheme" from the repo to preview and make GTK2-apps use my QT-Curve Theme. If I try other GTK2 themes like Crux and Clearlooks then they will most likely not properly work as they "absorb" settings from the GTK2 Theme if it comes bundled inside it's parent GTK Theme Folder. In other words for GTK2 Themes to properly work on GIMP, we have to remove the GTK2 folder from the main GTK-Theme chosen from the new KCM. Having the old option to have different themes for GTK3 and GTK2 was very handy for QT-CUrve users. Please bring it back.
I'm afraid setting the GTK2 and GTK3 themes separately is no longer supported, sorry. It was causing a lot of problems, and most apps are no longer using GTK2 anyway. For the ones that are, it's silly and embarrassing. GTK4 has been out for almost a year now! I understand that this may be painful for your GIMP theming. But really, GIMP should just finally release a GTK3 version. Or even a GTK4 version at this point. They've had like 15 years. Eventually, it has to be enough!
Gimp 2.99 (3 beta) is already a thing so it shouldn't be too long for it to start using GTK3.