Each user is able to change the theme of the homepage that will show when they are logged into the system. When logged out, or first accessing the site, they will see the default homepage theme you have set, but they are able to set their own to suit their tastes.
To change the homepage theme, select the settings tab at the top of the yellow box once logged in, from the list of available themes under the 'background' header, select the one you would like to use and press 'OK'.
You will notice that the theme in the top left box also shows further in the list, this is because the first one is the 'Default' theme, if and when we release a new default theme in the future, this will change.