dark theme

I think that it would be nice to provide a dark theme for the website (and the doc, later) using the `prefers-color-scheme` media query.

However, as bootstrap is defining colors of the theme in a lot of places and not using variables, it may be difficult to make sure that all colors are right.
The fact that the logo in the upper left corner is an image (with transparent background) also makes difficult to write a dark css theme as the text can't be seen anymore with a dark background.