Editor skinning is done via CSS. The Editor comes with a default skin, but you can extend or override this as needed.
The editors base class is a starting point for skinning the Editor. Include this file and use the skin file as a reference for your new skin (you can view the full contents of the base class here).
Once you have the base class in place, you can proceed to customize the look and feel of your Editor by working with the skinnning file. Starting with the Sam Skin version below is generally the fastest approach, allowing you to customize just those parts of the skin that will make your implementation resonate with the overall design of your application.
All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.