Question

I have an App_Themes directory, and also Master pages for a ASP.NET website.

Can I use 2 different themes based on the master page?

Answer 1

MSDN has an article about ASP.NET Master Pages And Themes

You cannot directly apply an ASP.NET
theme to a master page. If you add a
theme attribute to the @ Master
directive, the page will raise an
error when it runs.

However, themes are applied to master
pages under these circumstances:

• If a theme is defined in the content
  page. Master pages are resolved in the
  context of content pages, so the
  content page’s theme is applied to the
  master page as well.

• If the site as a whole is configured
  to use a theme by including a theme
  definition in the pages Element
  (ASP.NET Settings Schema) element.

In addition to the above you can see the section about Themes and Skins. You can change theme programmatically

Example from MSDN

protected void Page_PreInit(object sender, EventArgs e)
{
    switch (Request.QueryString["theme"])
    {
        case "Blue":
            Page.Theme = "BlueTheme";
            break;
        case "Pink":
            Page.Theme = "PinkTheme";
            break;
    }
}

But you cannot use two themes at the same time, that does not make any sense. You could however change the theme based on which masterpage is used.

To answer your question in your comment, yes you can have different themes for different sub-folders. This is from MSDN:

A theme setting in the Web.config file
applies to all ASP.NET Web pages in
that application. Theme settings in
the Web.config file follow normal
configuration hierarchy conventions.
For example, to apply a theme to only
a subset of pages, you can put the
pages in a folder with their own
Web.config file or create a
element in the root Web.config file to
specify a folder. For details, see
Configuring Specific Files and
Subdirectories.