Currently the look and feel of Confluence can be customized in three ways:
- Colour Scheme
Themes give you the power to customize Confluence completely. The same is true for layouts. But with great power comes great responsibility. You will have to know java, sitemesh or at least velocity in order to make customizations, which is not trivial and a skill most good web designers do not possess.
On the other hand, customizing the color scheme is fairly easy, but limited to a little more then a dozen pre-defined elements on the page. You can not add additional background images, change the size or hide/move elements on the page.
The Stylesheet feature will allow for overwriting all existing styles including styles in themes. That gives you the power to customize your favorite theme without actually modifying any of it's code. The only thing you will need to know is how to write style-sheets in CSS. This opens up the styling of Confluence to many web-designers out there.