The background color of the document can be set to dynamic, so it's feasible that you could use conditional statements to change the background color based on the user or user group. I'm not sure about the user dynamically changing the background, especially in AJAX/IE.
One method I've used in the past to allow end users to alter colours - both background and caption is as follows:
Set 3 variables - eg: 'vRed', 'vGreen' and 'vBlue'. Next either link them to three input boxes or as I prefer slider objects (set the scales to be 0 to 255 with steps of 1).
Then in the calculated colour box of either the background or caption enter the expression '=rgb(vRed,vGreen,vBlue)'.
That should allow users to amend colours to their hearts content.
As a note I also use this for quickly creating new documents for multiple companies; to change a .qvw to fit a new companies colour scheme you only have to change a few variables to match their corporate branding - very useful in the SiB world!
