How do I add background images/videos/sounds?
First create or edit a custom theme from the “Edit custom themes” option. For background images and videos select “File” to show or play a single file, or “Folder” to show or play a folder of files in random order. The folder may contain images or videos or both.
Setting sound files is similar.
The background is actually two layers: the images/video layer with the background color layer on top. So to see the media you need to set the backround color to be at least partially transparent. To do this adjust the horizontal slider control in the background color picker. All the way to left the background color will be totally opaque, to the right totally transparent, and in between the background will be a mixture of the two layers.