I installed Debian 12.8 with KDE a few days ago, and last night I was trying out various desktop customizations – and somehow got a setting that gives buttons on Oxygen Theme with Breeze Dark style and Breeze Classic color scheme a neon green or neon orange shadow. It’s quite jarring against the restful warm gray of the task bar or lock screen, and I can’t seem to find a way to get that shadow back to a variation of the button or background color. How can I at least restore the complete default theme and color scheme?
if going to the global theme and reapplying the default breeze theme does not fully restore everything, then one of those themes you were trying has left something behind as they are not always good at cleaning up after themselves.
from my notes:
## THEMES AND COLORS ##
#a global theme can add items to these settings, but may not remove them all upon uninstall
- Appearance > Global Theme
- Appearance > Plasma Style
- Appearance > Colors
- Appearance > Windows Decorations
- Appearance > Icons
- Appearance > Cursors
- Appearance > Splash Screen
- Startup and Shutdown > SDDM
#need to manually check each settings page, select the red trash can(s) and hit apply
I had already checked all of those pages, but none of mine have a red trash can (or any other color). I’m on Plasma 5; is that a Plasma 6 thing?
Have you look into the cache ~/.cache. You can either clear that folder or look for items belonging to that particular theme.
SImply emptying the cache could solve your problem.
You could … close SystemSettings. Rename ~/.config/kdeglobals. Restart SystemSettings.
And welcome to KDE Discuss.
Clearing the entire ~/.cache folder seems extreme – I’ve got a couple dozen folders inside there. I just emptied the plasmashell folder, figuring that was unlikely to mangle anything else, and that doesn’t seem to have changed anything.
I found ~/.config/kdedefaults/kdeglobals and renamed that; opening System Setting created a new one (reasonable to expect), selecting the global Breeze Twilight didn’t get rid of the neon shadows. 
it does no harm to delete the then entire ~/.cache folder
it will be recreated as soon as you start using things again… but sometimes things don’t get deleted from there like they should and so they can stick around like that last guest at a party that just won’t go home.