Both KDE Plasma and Gnome are reading the GTK4 user config from ~/.config/gtk-4.0/settings.ini
While Gnome keeps it simple due to libadwaita, on KDE Plasma - the GTK settings synchronization background service, will populate the folder with KDE Plasma specific files, to cover also the colors-scheme used, and also give the proper window class decoration.
On my end, all the GTK4 apps use it, and they get in sync with the changes i do.
That file should remain as it is by default, empty.
The actual issue is that Using GtkSettings:gtk-application-prefer-dark-theme with libadwaita is unsupported. so, in principle if you open blanket via Terminal you will see that warning. The proposal is: Please use AdwStyleManager:color-scheme instead. - but to be honest i failed to follow what is what …
Instead i would go either altering a copy of the .desktop file for such GTK4 app, so for example i place the one from Blanket in ~/.local/share/applications/com.rafaelmardojai.Blanket.desktop and change in it the line:
Or by altering the ~/.bash_profile with export GTK_THEME=Breeze so that way, when pam gets a new update the /etc/environment file is not overwritten …
Either way, if you also change the color-scheme accent color from System Settings, the GTK4 app needs a restart to catch up …