It uses some (at least in xhtml) non-existing attributes:
* The name, id, src, frameborder and scrolling attributes in the iframe element.
* The params attribute in the anchor elements.
I don't know if this is easily fixable, but since the silverstripe frontpage says that silverstripe is xhtml compliant, all the official (hosted on silverstripe.com) modules should validate out-of-the-box with the blackcandy theme.
Any thoughts?