Simply, you would have to modify either template to match the opposite. It may be easier to look around for a similar looking phpBB style
to match that of your Joomla! install and edit it until it suits your preference.
Additionally, there are some styles (RocketTheme, which you linked to, offers some I believe) for both phpBB and Joomla! that match.
In short: you will need to modify the template of one or another to reach the visual consistency you desire. If you'd like to do this on the phpBB end of things, I can move this topic to the Styles forum if you'd like.