Dynamically setting themes in ASP.NET

Actually, it must be set in Page_PreInit, it won't work if you try to change the theme in Page_Init.

The most common solution is to use a parent class for all your pages. This is a one-time only change and places the logic in the parent class. Instead of inheriting from Page you then inherit from, say, ThemedPage. Inside the class ThemedPage, which inherits from Page itself of course, you can override the Page.OnPreInit method.

You asked for "two lines", it's actually one if you remove the clutter. This is VB:

Public Class ThemedPage
    Inherits Page

    Protected Overrides Sub OnPreInit(ByVal e As System.EventArgs)
        Me.Theme = HttpContext.Current.Request.Url.Host.Replace(".com", "")
        MyBase.OnPreInit(e)
    End Sub
End Class

And instead of this:

Partial Class _Default
    Inherits System.Web.UI.Page

you now write this:

Partial Class _Default
    Inherits ThemedPage

That's all! A one-time search/replace and you're done. For completeness sake, here's the same (only the class) for C#:

// C# version
using System.Web;
using System.Web.UI;

public class ThemedPage : Page
{

    protected override void OnPreInit(System.EventArgs e)
    {
        this.Theme = HttpContext.Current.Request.Url.Host.Replace(".com", "");
        base.OnPreInit(e);
    }
}

Update: added VB code sample
Update: added C# code sample

Note: the theme must exist, otherwise you get an exception: Theme 'ThemeName' cannot be found in the application or global theme directories.. If you want a default theme or no theme if the theme isn't there, wrap it around a try/catch block and use the catch block for setting the default theme.