Overhaul the style picker in the footer

[BurntimeX]

The style picker is currently positioned absolutely in the boxesFooter element. This leads to problems, especially in the mobile view, if the available space is insufficient.

Ideally, the style picker should have its own element that it only shares with the footer menu, if at all. In this case, it should be positioned with display:flex. A button for selecting the color scheme could also be placed there.

This should probably be done together with a overhaul of the entire footer.

ref https://www.woltlab.com/community/thread/304585-fehlerhafte-anzeige-von-stil-%C3%A4ndern/ ref https://github.com/WoltLab/WCF/issues/5893