I’ve been wondering what sort of bug I’ve had in my WP installs for the last few weeks, and only now figured out what’s going on.The Turbo mode for WP is done through Google Gears. There’s a bug in the Turbo mode that will not allow you to change your blog’s theme. It works by not displaying the “x” (Close) or the “Activate …” options in the DHTML layer that opens up when you preview a theme.
Try it out if you want. Enable Turbo mode, then go to Design >> Themes and click on a theme that you’d like to preview and possibly activate. It’ll open as a full page instead of opening in a separate layer above the regular page, and the option to activate it will not display. In essence, you’re locked out of switching themes. You have to hit the Back button to get back to the Admin panel, else you’re stuck in a Live Preview mode.
This has nothing to do with file permissions, as I originally thought, or with corrupt theme files. No, it has everything to do with Turbo/Google Gears and the way WP implemented this. It’s a bug that needs to get fixed. The only way to enable theme-switching for now is to disable Turbo mode. After that, things work just fine.
This bug is present even in the latest WP version, 2.6.3. I hope it gets fixed soon.