Those apps you mentioned are not your typical modern GTK4/libadwaita apps, so naturally you can theme them. It's the GTK4 apps - and ones which use libadwaita specifically - that cause grief.

- Firefox: Uses its own custom rendering engine but interfaces with GTK3. It respects standard GTK3 themes.

- Thunar: Uses GTK3

- GIMP: Since v3.0, they now use GTK3

- LibreOffice: Uses its own visual framework (VCL) but has a GTK3 plugin to draw its window frame and menus, and it also respects the system theme.

None of these use libadwaita, let alone GTK4, so they're not good examples unfortunately.