You can change the background by simply selecting the free space around the email and going to the Style Manager -> Decorations -> Background and choosing from the swatch. The transparency is controlled from the scale under the swatch. This is for solid colors only.
If you wish to insert an image for your background, go to the function below and click on the plus sign. There, you can click on Image and choose one from the library.
For better looks, you have options on how the image repeats:
repeat - simply fits the image as many times as possible
repeat x - repeats it in only one line
repeat y - repeats it only on one side
no repeat - uses it just once
There are quite a few options for the Position of them image as well so play around and see what fits your idea best.
PRO TIP: How to add a gradient background
1.Create a gradient with a width of 1 pixel in a software of your choice and upload it to the email editor.
2. Then, set is an image background and repeat-x.