The designs used on this site
(meaning phpbb.com) are not released.
The designs used on area51 http://area51.phpbb.com
are what are included in the downloadable package, and are released under the GPL. You may use them in any way that is allowed under the GPL license, including a commercial product. But anything based on phpBB must also be GPL'd, so a commercial product based on software that must be released under strict rules might be a challenge.
GPL software and packages derived from GPL software must be released without restrictions. Meaning you can charge for the software, but you cannot prevent your customer from then turning around and modifying the software and redistributing it, for free or for fee. The copyright of the phpBB Group must remain intact as well.