Bug 451156

SUMMARY
I'm using the Breeze window decorations and the Breeze application style as well. Breeze-GTK is set as my GTK application style (using the System Settings module).

I find the default window shadows for Breeze to be much too large and dark - actually they're strong enough that they create a small amount of banding on my screen, which I find intolerable - and so I make it much smaller and lighter. Unfortunately, even though KDE seems to control the appearance of most GTK applications pretty well, changing the shadow settings doesn't do anything at all.

STEPS TO REPRODUCE
1. Make sure all instances of a Gnome application (the Image Viewer, eog, is my go-to for testing) are closed, and that you are using the Breeze window decorations and Breeze-GTK as your Gnome/GTK theme. This bug is reproduced under KDE, not in a Gnome session. 
2. Change the shadow setting for your window decorations. System Settings > Appearance > Global Theme > Window Decorations; click the edit "pencil" icon on the Breeze theme in the main window; use the Shadow tab. Click OK and make sure the changes are properly applied in KDE applications.
3. Reopen a Gnome / GTK application.

OBSERVED RESULT
No change to the shadows of the GTK application.

EXPECTED RESULT
The GTK application window should have exactly the same shadows as other windows under KWin - at least when the matching themes Breeze and Breeze GTK are used.

SOFTWARE/OS VERSIONS
Linux: Arch Linux kernel 5.16.11 x86_64
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

Graphics platform is X11.

I was able to work around the issue by manually changing the window shadows in ~/.config/gtk-3.0/gtk.css. Since (I think?) KDE manages these files when the Settings module changes the options for GTK applications, it should be possible to add handling for changing the shadows automatically.