I changed the header background by going to Appearance -> Settings -> Header and then uploading a new image. Checked "Repeat horizontally". This appears correctly as shown in attached file screendump_OK.jpg.
If I wait a while (a few hours or days), the background changes to a grey colour. See attached file screendump_bug.jpg . The configuration screen still has the new image file listed as the background, but it does not appear. Re-uploading the same file fixes the problem, for a while.
I noticed this several times in version beta11, but thought it was associated with the issue "Block headers are always Blue, no matter which scheme is selected". The block headers issue has been fixed in rc1, but this header background issue has not.
URL where screendumps were taken: http://serverhub.dward.us/~veganaustralia/agov-7.x-1.0-rc1/