themeSwitcher.php
When this plugin is enabled, a selector will be floated at the upper left corner of the browser window. This enables a visitor to select which theme he wants to use.
Theme selection is stored in a cookie. The default duration of this cookie is 120 minutes, changeable by an option.
No theme participation is needed for this plugin. But to accomplish this independence the plugin will load a small css block in the theme head. The actual styling is an option to the plugin. A theme may replace this css via the themeSwitcher_css filter.
Themes and plugins may use the themeSwitcher_head and themeSwitcher_controllink filters to add (or remove) switcher controls. The active() method may be called to see if themeSwitcher will display the control links.