Here you will find some informations. I think it is a good start.
Maybe someone knows some better source for reading.

Explanations:
http://leimu.drupalgardens.com/content/themechanger

Themechanger:
https://openrepos.net/content/rzr/themechanger

Source code:
https://github.com/matrixx/ThemeChanger

Theme settings:
http://vesuri.jormas.com/themesettings/