I have found it best to work off samples so, I have uploaded two.
This is a sample of a background used on Oasis/New Wikia skin. Note that before you can upload a finished png or jpg, it has to be under 100 kb. Yeah that is a small file size for such a large file. Note, the red will not be seen usually unless the article is zoomed out on. Note, you should know the the 6 digit color Hex you plan on useing on the background. The White is where the Article content/sidebar are.
Here is finished results if done right. (Note, it was uploaded through a new tool in the "My Tools" called -Theme Designer-.)