Dynamic theme CSS editing for designer?
You can try CSS Injector.

Project page says: 


  Allows administrators to inject CSS into the page output based on
  configurable rules. It's useful for adding simple CSS tweaks without
  modifying a site's official theme -- for example, a 'nighttime' color
  scheme could be added during certain hours. The CSS is added using
  Drupal's standard drupal_add_css() function and respects page caching,
  etc.