Document changes in custom CSS handling

Since e95286d90c8d0077ccc1117021c3856a628c0ef2 the custom.css file is
bundled with the main file, URLs are now relative to the parent css
directory.

Conflicts:
	docs/developers/customization.rst
The convention is to use cls for first parameter in classmethods
Bugfix: Change reference to unexistent cls object

Use self instead to access the classmethod.