Gimp is likely still using gtk2, which means you need a theme that supports gtk2. That’s probably old and un-maintained, since gtk2 has been End-Of-Life for a while now. gimp 3.0 is approaching though.
Is gtk not backward compatible with older version?
New main versions of software usually is not compatible with old versions. That’s one of the points of new main versions. You cannot load Qt 3 themes into Qt 6 either.
Gimp is likely still using gtk2, which means you need a theme that supports gtk2. That’s probably old and un-maintained, since gtk2 has been End-Of-Life for a while now. gimp 3.0 is approaching though.
Is gtk not backward compatible with older version?
New main versions of software usually is not compatible with old versions. That’s one of the points of new main versions. You cannot load Qt 3 themes into Qt 6 either.
No. GTK 3 was a breaking change, and so was 4.
Additionally, I think 3.18 onward doesn’t even support theming engines. As said, though, GIMP is stuck on GTK2.
If you’re having a lot of trouble, perhaps just go with the Flatpak.
They tend to use different theming engines each major version, so I don’t believe they are.