Version 2.0 of Mr Peer’s Theme Switcher is would be perfect, if it weren’t for the unfortunate breaking of WordPress’ built-in theme editor, causing all requests to open files for editing be redirected to the home page. The problem seems to be the theme parameter, which is used by both the built-in editor and Theme Switcher.
Fixing this issue seemed to be as simple as replacing all isolated mentionings of theme in the plugin source with a new, non-ambiguous keyword. I chose switchto, and the resulting code is available behind the previous link.
So, the parameter to switch themes with this modified version of the plugin is switchto=themename, and this eliminates the problem with the theme editor. Otherwise it should be identical to the original in functionality.