This page is using a free Darkly theme, courtesy of Bootswatch.

To use this theme with Msb3 Maven skin, use the following <theme> configuration in the site.xml site descriptor file:

<custom>
  <msb3Skin>
    ...
    <theme>bootswatch-darkly</theme>
    ...
  </msb3Skin>
</custom>

Find other theme options for Msb3 Maven skin here.