Theme isn't being applied to content page

[Guest]

I have a content page that is in a subdirectory that is restricted to certain
user groups. My website's virtual directory structure looks like this:
~\mywebpage
masterpage.master
default.aspx
~\mywebpage\App_Themes\Default
Default.css
~\mywebpage\AdminsOnly
adminOnly.aspx

The problem is that adminOnly.aspx is not themed. The header is exactly the
same as default.aspx, and intellisense agrees that the following page
directive should work:
<%@ Page Language="C#" MasterPageFile="~/masterpage.master"
EnableTheming="true" Theme="Default" %>

But it doesn't. The page is merged with the master page, but no style is
applied. What's going on? TIA.