Good catch! Has this term been removed? I can’t find it on the Basic Math site.

Another possible issue is the version of the GPL. If we accept the interpretation that all WP plugins and themes are derivative of WP, then I’m not sure the right licence is the GPLv3. WordPress is released under the GPLv2, and, as far as I know, there is no “or any later version” term anywhere in it. (This, of course, would apply to all themes and plugins, not just to Basic Math.)

Cheers!