First off, it's not really called theme builder but I don't like using the T word on this site, but when you get there you'll know exactly what it is.
Joomlashack by the looks of things are releasing a tool to allow you to create your own unique theme, in a way that you've never done before. Now, while I've not played with it yet, if it does all you'd hope for it to do this could be very impressive.
I'm particularly interested in knowing more about the 'Module Builder' option and as and when I finally get round to playing with it, I'll let you know how it works.
It's also worth noting that the support you get at Joomlashack is excellent and while this is very new, it will develop.
