If you won't create your own theme or simply want to change some CSS definitions, simply add an external CSS file to your servlet configuration:

<init-param>
  <param-name>externalCss</param-name>
  <param-value>../mycustomapp.css</param-value>
</init-param>

Put the mycustomapp.css into the root directory of your WebContent. It's also no problem to save the file in sub directories, e.g., …/css/mycustomapp.css, but don't forget to create the folder!

Hint

Use

jar!/package/mycustom.css

to load the custom CSS file from the classpath.