Considering how themes are implemented in phpmyadmin, where each theme has its separate files in the themes folder, each having css, js, images and the theme info,
I think theme generator could be deployed as a web app on the phpmyadmin website itself. Something like
www.phpmyadmin.com/theme-generator.
Anyone could create a theme and export it as a zip file with all these files in it, this could be directly extracted to the themes folder and they would be ready to go.
But, also considering the fact that phpmyadmin is moslty used in third party servers which users directly buy online from the host, there could be an upload theme option in the software which would upload and save the theme user creates.
Lakshya