I want to change the look of the sharepoint 2013 site programatically, I have tried setting up different theme but it didn't work.
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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();