I think it would be a good idea to be able to change your contact's background and colour scheme even if they are not online.

As far as I know, currently you can only change it when they are signed in which is a bit of an inconvenience.

There's a very easy way to work around that problem;
Load the main messenger window click on the Send an instant message (the minimizeable window at the bottom of the window) then click on the "Other" tab and then type their address, their chat window will be displayed (except with the contact is offline dialogue)...you can then change the colour of the contact's colour scheme...Hope this is some help to you...