Add another background image option to the home page banner theme #260
Description
Currently, only one image can be defined, for the right side of the banner. Theme design proposals I've seen also want to add background images to the left side (instead of just flat color). So I suggest making the theme a bit more flexible by by adding a background image option to the left side.
The only tricky part here is mobile. On mobile, the text area is not a nice square but a rectangle (based on screen size). So a background image might be cut off or not stretch the whole height. I'd solve this by ensuring the background image fades out into the background color (top or bottom, based on the flip option for mobile).