Support Light/Dark variants for all Editor Themes, and switch with the system appearance

[ian-twilightcoder]

MacDown should support light/dark mode in a similar fashion to Xcode.

1. Each Editor Theme should have a Light/Dark variant.
2. If a light editor theme is selected and the system appearance changes from light to dark, then the corresponding dark editor theme should be automatically selected.

[zoltanmaric]

I really like the markdown css from the Apostrophe markdown editor (unfortunately only available on linux). I realised that you can just:

• download the base.css and adwaita.css files from GitHub,
• copy them into the styles directory,
• reload the CSS list,
• select adwaita for your css

It looks very pretty and it will pick up your dark/light mode from system preferences: