A... unicorn?
The only way of adding any decor would be by adding it to the footer of the forum, which is right at the very bottom near where the copyright text is. You can't add any down the sides unless it's a repeating texture because of the way the forum stretches to accommodate the pages.
I really don't see the "plainness" you're talking about. It's a LOT less plain than plenty of websites, atleast the background is a gradient compared to the plain white you get on plenty of other websites. Take YouTube for example.