BlackBerry 7 has been out for what seems like forever now, but we still have no official theme builder. Fortunately there is a work-around that theme developers have been using to get the job done. All may not be lost however, as @BlackBerryDev stated on Twitter that Theme Studio 7 is "en route".
@tw820 You're not alone. And Theme Studio 7 is en route and we are looking to release ASAP ^BZ
Fingers crossed that we'll see something come of this sooner rather than later. Maybe a release at BlackBerry Jam later this month? Time will tell. If you're a theme dev and want Theme Studio 7, give a shout to RIM in the comments.