When attempting to debug a theme on a deployed site in VS2012 I get a
The type or namespace name 'MyTheme' could not be found (are you missing a using directive or an assembly reference?)
error from all the MyTheme views that reference MyTheme. The debug log shows
2013-09-20 10:47:31,582  Orchard.Environment.Extensions.ExtensionManager - No suitable loader found for extension "MyTheme"
I can recover the site by moving the MyTheme.dll into AppData/Dependencies but that prevents me from debugging the source code.