@expvt GX Cleaner shows up in the sidebar if enabled, never used it myself. You can click the icon that looks like 3 bars at the end of the address bar to change themes, or also right-ckick on the speed dial background and select Change wallpaper to get to the same panel (Easy setup). Alternatively you can right-click on an image and choose "Use image as wallpaper" from the menu. The Opera add-ons site also includes some animated themes; if you select "Get more wallpapers" it'll show them to you.