well I was thinking you could just share the pictures here
technically it might be possible to do it on a phone, if you can get an app that can save images in 16 bit format, but a theme is more than just top and bottom screen images, there are a lot of other parts to it like icons, button graphics, colors, shadings, font/text size, that you would have to figure out, and the layout of it isn't very intuitive to modify without a program that can preview your changes for you. I don't think a program like this exists on mobile
if there is an existing theme you like and that you would use as a base, and just replace the top and bottom screen pictures, that might make it easier. I guess you could look for other themes online with preview pictures, and see if you find something that would look nice with your background images. otherwise if you have an idea in mind of what you would want the rest of the ui to look like, that might be workable too