My twitter feed (full of people in the WordPress community after meeting a ton of people at WordCamp Boulder last weekend) unexpectedly caught fire this morning on the #thesiswp hashtag. I had no idea what the fuss was about, but I wasn’t surprised when I read into it: the item in question is Thesis, a robust premium WordPress theme that costs a minimum of $87, and whose source is under a closed software license.

The debate and confusion is really about the licensing status of custom WordPress themes. WordPress is covered by a copyleft license which requires that works derived from the software be covered by the same free, open source license (specifically, GPL v2.) But “derivative works” is a pretty vague concept, and can be interpreted in many different ways. That’s why WordPress founder Matt Mullenweg wrote the Software Freedom Law Center, some of the most experienced legal experts on libre software issues. They provided a rather comprehensive interpretation of the issue:

“In conclusion, the WordPress themes supplied contain elements that are derivative of WordPress’s copyrighted code. These themes, being collections of distinct works (images, CSS files, PHP files), need not be GPL-licensed as a whole. Rather, the PHP files are subject to the requirements of the GPL while the images and CSS are not. Third-party developers of such themes may apply restrictive copyrights to these elements if they wish.”

This falls in between WordPress developers’ wish that the whole community support libre software and Thesis’ completely closed license. Theme PHP must be GPL-compliant, but the graphics and CSS may be licensed otherwise.

As someone who makes custom themes for clients, I am familiar with the feelings of apprehension about open sourcing some of your work – often done for a client who neither knows nor cares about the finer points of free software principles. The common fear is that by giving away your code, you also give away your business model. This couldn’t be farther from the truth. (Unless your business model depends on every customer abiding by your copyright – a foolish strategy in light of how easy it is to pirate web app source code, not to mention an overvaluation of the originality of your source code) (UPDATE: WordPress’ own Jane Wells points out that it’s even less complicated than this for custom theme work, as you only must publish your source under GPL if the theme itself is publicly distributed.)

The truth is that many companies comply with the GPL, retain their trademarks and licensing rights (including WordPress theme graphics and CSS), and do so to great profits. Google, Apple, Facebook, Red Hat, Novell, and countless others make their GPL source available – as do many other WordPress premium theme makers. You can sell themes as long as your PHP complies with the GPL. Pirates can easily copy the rest of your theme regardless, but embracing the GPL not only complies with copyright law and the license terms, but it supports the ideals that made WordPress possible, and makes the whole community project stronger for everyone. And you don’t have to go out of your way to be financially sustainable while doing so, either. Novell and Red Hat sell their entire OS open source under the GPL, the Mac OS X kernel and UNIX userland is open source, so there is no reason why a WordPress theme can’t be both GPL-compliant and profitable.

In short: