application: follow upstream dark style preference changes
Use libhandy's StyleManager to follow the dark mode variant used by libadwaita and GTK4 apps. This allows the app to change its style based on GNOME Settings toggle in Appearance panel.
Related https://gitlab.gnome.org/GNOME/Initiatives/-/issues/32
Marked as Draft since I haven't tested this one yet.
Edited by Pablo Correa Gomez