Dialion:
How do i set my theme?
There are several steps on how to use Themes in ASPNET so i would suggest you to read on this article
http://www.odetocode.com/articles/423.aspx
BTW: When do you want to change the Image Background on the Page? I think you can handle or set the image URL dynamically in code behind part... For example if you wanted to change the Page background on Button Click ... all you need to do is give the page <Body> element an ID and set the runat="server" attribute like
<body id="pagebody" runat="server">
Then on Button click you can change it like this
this.pagebody.Attributes.Add("style", "background-color:Blue;");
// IF setting the ImageURL then you can have something
this.pagebody.Attributes.Add("style", "background-image: url(~/Image2.jpg)");
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