Where does WP pull CSS from?

I have searched all the files (in my current theme) to find a hard coded path for the css files, but could only find.

<link rel="stylesheet" type="text/css" href="<?php bloginfo('stylesheet_url'); ?>" />

Is there a way that I can pull a different CSS file? What I want to do is change the color theme any time I change my main image. I know this is probably just as easy as to go into my style.css and change the colors but if I want to revert back to a different color then I have to change all the codes.

For example, my main website is calling the css file “khaki_red.css” How do I have WP call the same file?

If you need more details let me know – Thanks