Overrides installed Gnome & Gnome-shell-themes

[tristanRW]

#The issue: Currently Stylix is overriding a lot of configuration that it does not provide but that a user may have set outside of it. For example: The way gnome-shell-theme is set overrides existing changes outside of stylix. #The proposal: I suggest adding logic to "modules/gnome/theme.nix" that exposes an option for the user to provide a theme-package to use as a base for further changes. A similar change can be made in "modules/gtk/hm.nix" Where the gtk-theme is always forced to adwaita. Exposing these settings as Options or as Overrides would improve QOL a lot.

[danth]

We may need to implement colors differently depending on the theme used, especially for GNOME shell where the theme is basically just a CSS file. This is why it is currently locked to the default theme.

#203 could be related.