[META] Sprint CSS cleanup

There's a lot of closed / old issue for drupal core CSS cleanup. Lots of changes has been done to the core modules, and it's time for some more clean up on the markup and CSS. Also provide try to provide a better class names and good markup with smacss principles.

Main principles:

CSS architecture (for Drupal 8)
CSS file organization (for Drupal 8)

Issues covered:

#1969132: Clean up the CSS for Color module
#1969124: Clean up the CSS for System module