Clarify GPL license

[gvolpe]

Hi,

Could you clarify whether by GPL3 you mean GPL-3-or-later or GPL-3-only? Just GPL is ambiguous: https://gnu.org/licenses/identify-licenses-clearly.html.

The GPL-3-or-later is strongly recommended by GNU Foundation: https://www.gnu.org/licenses/gpl-faq.html.en#VersionThreeOrLater

This is important to distribute this theme in some Linux distros such as NixOS.