Stefan,
For the backend CSS, a lot of is governed by some standard Kentico CSS to make sure it's responsive and stuff. You can set the "Editor CSS" which allows you specify a particular stylesheet to be used on the "Pages" application, which may or may not help. For the standard Kentico design, you can find a lot of info from this article, which may lead you to what you are lookign for:
http://devnet.kentico.com/articles/kentico-8-technology-design-system
Bryan Soltis
Kentico Technical Evangelist