If you want to tune the way your site looks, you can use a custom skin to provide your own CSS stylesheets. If that is still not enough, you can even tweak the output templates that Maven uses to generate the site documentation.
For an in-depth discussion of site customization, please have a look at Maven: The Definitive Guide, Chapter 15. Site Generation provided by Sonatype.
A skin contains the following elements:
A skin is built like any other JAR - with a packaging of jar. No additional plugins are needed.
Resources should be put in the normal src/main/resources directory.
Once the JAR is built and deployed, it can be used by projects.
If you are interested in constructing your own CSS, it is recommended that you copy the file maven-theme.css from Maven Default Skin and modify it to suit your needs.
The format of the Velocity template is currently out of scope for this document. To attempt this, start by copying default-site.vm to src/main/resources/META-INF/maven/site.vm in your project and then modify it to your needs.
A list of links to skins that can be used as inspiration can be found on the wiki.