Add stylesheet variant for Bootstrap

[sveetch]

This would be helpful in Django admin which stands on Bootstrap with CSS variables and enable the browser "dark mode".

Currently the stylesheet hardcode its own colors, it is safely usable in dark mode but always with a "light" look.

A proper solution have to be found to easily use the right stylesheet from widget implementation.

[sveetch]

Worth to notice than fileinputbutton CSS is build from a Sass source included in SmartMedia repository, frontend developer that may need to customize it may be able to copy and include it in their Sass sources.