As you are using .Net 1.1, and don't have the option of theming, the easiest way would be to have multiple stylesheets, and use the QueryString variable to determine which stylesheet to link to. In the <head> section of your aspx page, add a standard link element that links to your default CSS file. Give it an ID and mark it to runat="server". In your code-behind file, make sure the link element is declared as type System.Web.UI.HtmlControls.HtmlLink along with the other page controls. Then, when the page loads you can simply check the QueryString to see which stylesheet to use. If you have a valid stylesheet then simply set your declared link elements href property. Hope this helps...
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