Uploading a custom background is easy. Go to the "Appearance" tab under your paper settings. Paste the URL of the image you would like to upload and save. Your image will load automatically.
If you want to get rid of it, go again to this tab and click on the red x on the top right of the picture, it will then be deleted immediately.
Please note that the maximum size for the image is 1280 x 800 pixel and 800 KB.
You can also choose a color for your background here. Just click on the box and you will see the whole color possibilities. If you do not want it any longer, you can delete it by clicking on "Revert to default".