Environment vaiables

Milsko programs can be optionally configured at runtime using the following environment variables:

MW_LIGHT_THEME/MW_DARK_THEME: ignore the system's appearence settings and either use light theme or dark theme respectively .
MW_BACKEND (Linux/X11 only): set to either wayland or x11 to force which display protocol Wayland uses
MW_FORCE_CSD (Linux/X11 only): set to force the wayland backend to use CSD instead of SSD