Use UserCSS vars and config dialog for built-in UI styles

[tophf]

The problem is that currently we can't just freely experiment with small things in UI because we want to avoid aggravating people who may dislike the changes so we would have to expose an option, but we only have our main options UI that should contain behavioral/functional options, not a hundred of UI tweaks.

The solution is to reuse usercss vars feature which we can expose in the usercss config dialog. We'll use the default preprocessor with its --css-vars so we can keep loading our css files directly without compiling them (the values will be added in a separate style element). By doing so we will be able to offer more tweaks and choices for various trivial UI things.

Some existing options to be converted:

• Popup width (in pixels)
• Styles before commands
• New manage UI layout
• Favicons in applies-to column
• Grayed out
• Number of applies-to items

Some of these options can't be converted to pure CSS so naturally we'll keep the current JS code for them.

More stuff to add:

• dark theme, see https://github.com/openstyles/stylus/issues/1160#issuecomment-773749889.
• applies-to position (above/below the code) in classic editor
• global font and size
• code font and size
• vertical multi-column flow