Remove background rule with no image

The intranet theme doesn't seem to use the background images of the Web Usability theme. Note that this isn't currently mentioned in http://wet-boew.github.com/wet-boew/demos/theme-gcwu-intranet/index-eng.html#hdrchgs

Fixes gh-905
Merge pull request #912 from nschonni/patch-1

Remove background rule with no image