TinyMCE is a free WYSIWYG editor with two visual components - themes and skins. TinyMCE themes are very complex to customize but you can make your own TinyMCE theme. If you just want to customize the look & feel a little bit, you're better off creating a TinyMCE skin.
Customizing TinyMCE themes is easy to do using skins. Each TinyMCE theme can have multiple skins that define custom CSS and images for the WYSIWYG editor interface.
The default theme is called "advanced". The advanced theme comes with four skins: default, 02k7 (a Microsoft Office like skin), o2k7 Silver, and o2k7 Black. You can see how each of the skins looks on the TinyMCE examples page.
If you have been searching for TinyMCE themes, good luck. There aren't any. Well, not many. The documentation for creating a TinyMCE theme is extremely outdated and doesn't apply to the newest version. Fortunately I wrote up a guide on creating a TinyMCE theme, but it's a bit of work. When I learn some more about the process I can hopefully contribute to the documentation.