Change Default Sharepoint site theme

 When you create new sharepoint site using any Template (Team site,Blank Site , Blogs etc.)  default it sets blue colored Theme (i.e Default Theme).

We can change this above default set them to any themes provided by SharePoint (Verdant,Petal etc) .
using below steps.

1. Create layout page (application page) as changeTheme.aspx .
2. In Code behind of page on page load write below code as

protected void Page_Load(object sender, EventArgs e)

{
SPSecurity.RunWithElevatedPrivileges(delegate()
{
SPWeb web = SPControl.GetContextWeb(Context);

web.AllowUnsafeUpdates = true;

web.ApplyTheme("Verdant");

web.Update();

web.AllowUnsafeUpdates = false;

Response.Redirect(web.Url);

});

}

3. Delpoy this page to layouts Directory .

4. In ONET.XML of Team site template add <ExecuteUrl> tag Inside <Configuration> node as shown

<Configurations>

<Configuration ID="0" Name="Default">
<Lists/>

<Modules/>

<ExecuteUrl Url="_layouts/changeTheme.aspx" />

</Configuration>

<Configurations>

5. Add same <ExecuteUrl> for other templates ONET.XML and Restart IIS.
6. Create Site using Team Site Template ur Site will get created with Theme set to Verdant.
 
Thats it !! Done ...