As we know, CKEditor is used inside webpages. In fact, the CKEditor UI and the skin CSS files are all loaded inside a page that is out of the editor and skin control. This means that the skin can have interference from external CSS present on the page.
Let's suppose a site likes red links. By simply adding a{color:red;} in a CSS file would be enough to make the toolbar labels or dialog tabs of the editor red.
To workaround this problem, a skin developer must include "reset styles" that will define defaults for most of the styles that could impact on the editor interface. Check the reset.css file in the Kama skin. It is a useful base for it.