ELAG 2013 Workshop on customizing the DSpace XMLUI Mirage interface.
The workshop first explores what can be changed in CSS, exploring the different functions of the style.css, base.css and reset.css files.
It then highlights where all of these files can be found and where you need to deploy your own customizations.
Digging down an additional layer, it is explained how XSL can be modified to remove or change entire blocks of functionality on a page.
The key learning here is that you can alter the representation of whatever comes in through the DSpace DRI using XSL. However, if you need to include additional data or other DSpace info, you have to make sure that it appears in the DRI first, before you can start transforming it with XSL.