Here is a Background_Template that I created using paint.net to help make custom backgrounds for the lean theme layout with just a few easy steps.
Follow these steps to make you own custom BG.
1. Download paint.net Obvious...

2. Open the template that I uploaded here.
3. On the top right corner open layers or hit F7. This should open the layer box if not already open.
4. With layer box open make sure background is selected. You can ignore the other layers unless you want to create more for more objects etc.
5. Ok, now find some wallpaper images you would like to use for a background theme. Once you find something you like open it with paint.net. Now, notice at the top you should have 2 small windows with images. This is the image you just opened and the original template.
6. Now, go to the image menu or Ctrl+R and re-size the opened image to 1024x768. After re-sizing image do Ctrl+A then Ctrl+C
7. Last step, Click on the template window then Ctrl+P. You should now have your own background.
8. Save as type png, and be careful not to overwrite the original pdn template as it is the default. From here you can just rinse and repeat over the same background.