First Make it so that your whole page is filled black
Step 2: Designing
Now click on the square tab and choose the red color and make some kooky rectangles
Step 3: Now add some blue
Now change the color to blue. make some more crazy rectangles
Step 4: Green!
Now for a little green.
Step 5: Purple...or any other color
Now you can really get creative and have as many colors as you want. here is one of my designs. when you are all done, go to edit, choose select all. there should be a border around the design. now go to file and select set as background and walaa!