Enable dark mode

Closes #953 (closed)

Short description

A lot of CSS has been refactored to easily support CSS prefered-color-schemes. All regular SCSS (except things in the admin, or some special exceptions, such as signature fields) should now use variables (like $preset or $preset-contrast, $background-color, etc.). Then in _variable-usage.scss files specific to the chosen color scheme, those variables are mapped to real colors in ~_variables.scss`.

Therefore in _variables.scss we define all settings specific to our identity, and in _variable-usage.scss we can use those settings to create a color scheme.

Steps to use this feature

1. Visit the website, from a agent that has prefered-color-scheme: dark