Justice? You could always sue the thief. But, whether or not you would win would come down to the judge's interpretation of the GPL since there is no direct language protecting designs from the GPL. You would be fighting an uphill battle, for sure.
Notice my use of the word design and not theme. By strict definition of the GPL, themes become GPL when they include Moodle core code. But, can we own the CSS
and artwork that make up the designs?
For example, imagine that I make a custom Moodle theme for General Motors so that they can retrain their workers to make solar-powered automobiles. They want the theme to match their existing website. Since this is something I frequently do, I make them a theme that seamlessly matches their site. Two months later, Left Foot sees the Moodle theme, harvests the HTML, CSS, and images, and uploads an identical theme (sans branding) to the Moodle Themes database as GPL. He calls it GMtraining. He gives everyone involved credit and is totally upfront about everything that he did. After all, if it was in a theme folder, it was GPL.
Can General Motors take action against Left Foot for stealing their design IF it has included in a Moodle theme?
The answer is "Of course." Anyone can sue anybody (in the U.S. at least). But, would General Motors win?