Removed some unnecessary 'use' imports
Temporary folders created by tests should be removed
Refactored the CSS styles for all themes and formats

  * Previously there was only one HUGE style.css.twig file.
    Now this file is splitted into several smaller CSS files
    (normalize.css.twig, typography.css.twig, navigation.css.twig, etc.)
  * The 'Bright' theme has been deleted because it wasn't polished
    enough. Soon we will introduce some new and useful themes.