Finish CSS segregation

This MR splits the default theme in several chunks that make them re-usable by other themes (the themes often share some parts, like the header for example).

For example the light theme now imports some parts of the default theme instead of importing the whole default theme and overriding some elements.

I did not modify the dark theme yet but I think we can still close #650 (closed) and improve the dark theme later.

This should not affect any theme visually, but this was quite a long work and I cannot guarantee it, if you could test it too I'd be glad.

Fix #650 (closed).