Started on theme support

  - Add darker() and lighter() functions which does the opposite if the
    $backgroundColor is lighter than the $fontColor

  TODO:
  - public/sass/_gist.scss
  - public/sass/_selectize.scss (this might go away)
  - public/sass/_landing_page.scss


Note! This branch need to be tested before we merge it into master. The idea is that you should be able to make a new theme just by touching _variables.scss.