How to switch different themes conditionally and automatically based on path

For instant,I like to use Garland in the admin section such as /admin/* WHILE I could apply my customed theme in the other section, I have learn that some code could meet this condition, I like to get some idea about what these code should be and where to place these code, it's so kind of you to give me some instructions.

My current version is drupal 6.x.