Cannot change themes after hard reset

Maybe something easier to check :

find ~ ! -user $USER

( copy paste this command as is )
In normal circumstances, that command should output nothing, and it’s ok.

Else it will list files and folders that no longer belong to you and should be reclaimed.

Do you happen to play with sudo commands ?

Or, bad, do you happen to launch « graphical » applications from terminal but with sudo ?
Bad things like sudo nemo or sudo gedit or sudo any_windowed_app ?