Dear Feva and Chris,
Ok, firstly I'm not a lawyer but a software engineer, so this is my 'understanding'.
GPLv3 allows you to sell https://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowMoney.
Because of the way that Moodle is licensed with GPLv3 and with themes being a 'plugin' then https://www.gnu.org/licenses/gpl-faq.html#GPLPlugins applies. And thus the theme must be also GPLv3.
But... there is a bit of a grey area when it comes to CSS and the artistic elements of a theme which are not really 'code' and thus GPLv3 does not cover them. This is how certain sites can 'sell' themes and impose certain restrictions upon them, some elements must be GPLv3 and you can reuse them, but the rest has the restrictions. But I understand that by publishing a theme on Moodle.org, no such restrictive licensing is used on those elements.
Thus therefore any commercial site selling Moodle with Fordson could do so but then the person receiving it could then make it available for free if they chose to do so. Clearly if there are other plugins on the site, then their licenses would also need to be checked.
As another example, the toggle icons in the Collapsed Topics course format could be restricted via a copyright and not GPLv3. However as I created them myself then I have chosen to make them freely available.
This has all been discussed before in posts on this forum and the developers forum too (I believe), you'll just need to search for them.