Okay
This is a transparent image - As you can see, the image is exactly the same in each case, but the top part of the image is the same colour as the cell it is contained in.
Now, let's apply that to the sidebar - Same as before
Now, we have 2 options - We can either add a design on the sidebar image itself and add the whole image as a cell background ( like this ), or make the sidebar image the background of the cell (like you do now) and add the design in the cell itself ( like this ). View -> Source on that page might make it easier to understand too.
