I want to change the look of the sharepoint 2013 site programatically, I have tried setting up different theme but it didn't work.
First you need to understand themes in 2013.
Start here for the basics
Then here to grasp how to make a custom one
Then you need to understand where and how they are set from this, then you can alter it., something like this:
ThmxTheme oTheme = ThmxTheme.Open(siteCollection,"_catalogs/theme/themename.thmx"); oTheme.ApplyTo(oWeb, false); oWeb.Update();
There are many ways of achieving this. It can be done by creating custom masterpages, custom CSS files etc. etc. etc.
It all depends on which part of the SharePoint 2013 you want to change.