In the admin dashboard, I go to Appearance/Themes/Add New, and search for themes. I get the list of themes with "Install" and Preview" as options. If I click either one of those, the screen darkens, but nothing else happens (the theme install or preview windows do not appear in the overlay, in other words...the background only darkens in preparation for it).
Browser is Google Chrome. OS Win7-64 bit.
Let me know if you need more info to troubleshoot and I will post it here. Thanks!