How can I change active theme for specific paths or inside module? I'm working on an online game engine website where users can create games online and other can play it.

All pages are displayed in normal theme except game play page which should be in full screen mode inside a minified version of main theme.

I know themekey is useful and I can change theme with it. but I don't want use a complex module for this purpose. is there any API function for changing theme inside module on specific paths?
There are several modules you could use besides themekey:


Sections
Theme Rules (There is a 7.x branch, you'll probably need to check it out with Git)
Theme per path


Some of those are more focused for your task and don't have the overhead Themekey has (although it is a good module, just a bit heavy perhaps).