Some notes about stylesheets:

* NOT every directory in css/ is considered a style to be chosen. The available
  stylesheets are hardcoded in at least 3 places:
  - SL/Auth.pm:read_user
  - SL/Controller/Admin.pm:init_stylesheets
  - bin/mozilla/am.pl:config
  yes it sucks. change it, if you like.

* If a stylesheet is requests from controller code, it is searched in this order:
  - css/<stylesheet_of_current_user>/<path>
  - css/<path>
  - <path>

  so common libraries can be put higher here, to be included by any stylesheet

* 'kivitendo' is used as the fallback style if no style was found or the style
  chosen was invalid.

* Pay attention to the position of relative image urls. Most should not be
  included in css anyway but should use the generated icon maps. For the rest,
  a couple of ../ may be needed to get the right path.

* If you edit another styles, these are things that are known to get broken.
  Be sure to test them thoroughly:
  - tabs
  - calendars
  - all menus
  - autocompletion (parts/customers)
  - charts of accounts
  - multiselect
  - ckeeditor
  - jquery-ui widgets (i hate those things)