feature: light/dark mode switch setting

[maximilien-noal]

Should fix #55

See documentation:

https://docs.avaloniaui.net/docs/next/guides/styles-and-resources/how-to-use-theme-variants

"Should" because I still can't build the latest master on Linux. (See the latest update of #52 )

[maximilien-noal]

This doesn't work. Back to the drawing board.