Split css

This MR moves the app layout (only the app itself, not each section yet) from the default theme to the base CSS. This also moves some other parts to dedicated files (more because it is needed to further use @import than for the files size).

The CSS itself (beside the last commit which removes dead code) is not modified. I only prefixed some parts from the nav to make it easier to read.

Nothing should change visually, this is just the first part of the "midway solution" for #650 (closed).