Hello, thank you very much for your support and kind words! We didn't expect anyone needing to export output with solid colors as background, so thanks for reminding us that. It should be simple enough to be added in the next nightly build; hopefully we can push one in a few days as there's a little bit of personal issues at hand right now.
As a temporary workaround, you can make a solid image with 640x1280px in dimension (the same as Mannequin's canvas), and then use that image as background using the available menu in the 'Art Style' sidebar. Make sure that 'Include when Export' option is active!
After this, the exported image will also include your chosen background image (whether it's PNG or PSD):
Hopefully this helps, and don't hesitate to let us know if you still has problems!