Some other SE groups I'm part of, eg. Bicycles has a colorful themed background behind it. Presumably they can only be a good thing, making the site look more professional and retaining users. How would the gardening and landscaping group go about designing and choosing a background?
All SE "Beta Sites" are displayed in the Blueprint theme. When the site graduates then it can produce and display it's own unique design. If you would like to get involved, this Meta Discussion would be a good place to start.