@raymerjacque because obviously you don't understand.
It already has GTK and QT theme, it is styling. Drawing titlebar is not theming. Client side decorator has nothing to do with theming !
Do some reading !
GTK theme or not, does not affect client side decorator
Only Opera has that border? is not a theme. Brave, and Chrome is the same, does not use titlebar.
Brave, has option to set "classic" or "GTK" but does not affect the titlebar. Titlebar is a separate option in Brave.