Use as much CSS variables as possible

[NeOMakinG]

Mainly on Hummingbird-specific components, we would want to be able to customizes colors, sizes and font-sizes using CSS variables, without the need to implement a bundler

This issue can be closed when every components of the theme are customizable as much as possible

We are aware of the fact that bootstrap doesn't support CSS variables for every component yet, we need to update it when https://github.com/twbs/bootstrap/issues/35319 will be closed