I originally upgraded to WP 3.5 and afterward any new themes downloaded would not activate or give me a live preview. The only themes that work are the one currently active during the upgrade and Twenty-eleven as it was also downloaded but not active. Interestingly, even Twenty-twelve will not activate since it was downloaded after the upgrade.
I even installed the 3.5.1 patch from my ISP but no go.
I have contacted the authors of two of themes but they have no answers either.