Ability to set GTK_THEME

[VarLad]

@torikulhabib While you might not want to use libadwaita #156
The current theming is hardcoded in Gabut. Can you make it so that Gabut works with GTK_THEME?
That way, anyone can use any theme they want, for example, to use adwaita theme, we can just launch GabutDM with GTK_THEME=Adwaita:dark gabutdm
Most GTK4 apps can do this already. Is this feasible for Gabut?

Thanks for taking time to make Gabut by the way :)

[VarLad]

Now eagerly waiting for a release! 🐱