Apply specific theme to all sharepoint site pages programmatically

Brief steps for applying specific theme to all sharepoint site pages programmatically.

-> Create a C# console application
-> Add Microsoft.Sharepoint.dll reference (If you don't know how to do this see Sharepoint Hive FAQ in below list of FAQs)
-> Write below code:

Using Microsoft.Sharepoint;

Public static void Main(...) {

SPSite sc + new SPSite("sitename");
foreach (SPWeb web in sc.AllWebs)
web.ApplyTheme("citrus");
}

Very easy!