[theme-reviewers] Licensing Issues with Your Themes in the WordPress Theme Repository

[Otto]

AFAIK, the only requirements for the theme and plugin repos with
regard to license are GPL-Compatible. No version is specified. The
license of WordPress itself is irrelevant to this requirement.

So GPL (any version) is fine. As is Modified-BSD. Or the X-11 license.
Or anything listed here:
http://www.gnu.org/licenses/license-list.html#GPLCompatibleLicenses

-Otto