I was just fiddling around with it, and found that disabling the front-end admin bar changes fixed my problem of the background being changed in the user-end.
The header shrinking issue i’m afraid i can not help you with for it seems completely random to me.
-Edit: After disabling the menu width slider by collapsing the menu to icons only, i seems to have no more header shrinking issues.
-Edit: found another bug, disabling the footer does not seem to work ??