1. Download the attached doc.zip.
2. doc/api contains the stylesheet and images for Javadoc.  Extract them into the existing Javadoc output to overwrite the existing stylesheet.
3. doc/xref contains the stylesheet and images for cross-reference report generated by maven-jxr-plugin.  Extract them into the existing Xref output to overwrite the existing stylesheet.

To apply the stylesheet with maven-javadoc-plugin, you need to add three configuration properties in your pom.xml:

...
      <plugin>
        <artifactId>maven-javadoc-plugin</artifactId>
        <!-- Note: Older versions will not work. -->
        <version>2.6.1</version>
        ...
        <configuration>
          ...
          <stylesheetfile>${basedir}/src/javadoc/stylesheet.css</stylesheetfile>
          <javadocDirectory>${basedir}/src/javadoc</javadocDirectory>
          <docfilessubdirs>true</docfilessubdirs>
          ...
        </configuration>
      </plugin>
...

In the example above, the images are located in the ${basedir}/src/javadoc/resources directory.

Unfortunately, there's no built-in option that makes the maven-jxr-plugin to copy the images.  As a workaround, you can modify the stylesheet to refer to the Javadoc resources using relative path (e.g. url(../javadoc/resources/*) instead of url(resources/*)).