Reimplement site background in Shubara

[voltangle]

Rn it's done quite crudely in MediaWiki:Citizen.css, should be reimplemented in shubara with:

• multiple versions for different screen sizes
• a compressed source image
• proper support for light and dark mode

[voltangle]

Just checked how to do it, there is no hook that will allow to do this
The only way is to actually patch the skin (Citizen) itself, just manually add two <img> tags for light and dark mode and use stuff like display: none; to switch between them
These two <img> tags have to patched into Citizen skin templates, more exactly - templates/skin.mustache