The new theme support in ASP.NET is pretty cool. Although personally I don\'t think they took it far enough. When you design your page you can add an attribute to the @Page tag that states which theme to use when rendering this page, this gives the web site designer some flexibility in that they can change the look and feel of the site with a simple change. The simplest way to do this is with a single global change in the web.config
<configuration xmlns=http://schemas.microsoft.com/.NetConfiguration/v2.0>
<system>
<pages theme=”MyTheme"”/>
</system>
</configuration>
For me though the cool thing about themes is to allow the user of the web site to select the theme they would like to use. As far as I can tell there is no direct support in ASP.NET to do this for you. So I set about writing a component that could do this. So what we need to do is. For each page as it is about to be rendered change the theme used for that page select the theme based on the user preferred theme
In order to intercept the rendering of each page at the appropriate point you can provide a handler for the Page PreInit event, inside this handler you can add the appropriate code to change the theme to use for just that page. In order to hook up to this event I decided on a design where if you wanted a page to have the user selected theme then instead of creating a WebForm that derives from System.Web.UI.Page you actually derive from my new UserThemedPage type which ultimately derives from System.Web.UI.Page. This can be simply done inside the forms Code view, Inside the constructor for UserThemedPage I add the necessary event registration code. Thus when the WebForm is created the code  is in place to handle the PreInit event and apply some policy to determine what theme to actually use. For the second part we need to provide a way for the user to select a theme and also for the theme to be retained across multiple pages and hopefully multiple visits to the site. I didn’t really want to introduce the need for non anonymous access and store the users preferences in a database, so I opted for the use of cookies to store the user\'s prefered theme. Since not every user wishes to enable cookies, I also opted to store the theme in the session store allowing users to retain their preferred theme for the duration of the session, but would need to reselect it on re-entering the site.</p><p>As for selecting the theme to use, I did not want to provide the actual UI for this but simply a method that would allow simple iteration of possible themes to use and then a simple set/get property interface for setting the chosen theme. The iterator simply returns the set of themes that are defined for the current application, its then up to the chosen web app to decide how to ask the user for their chosen theme. So now in order to have a user select their own theme and have it persist across multiple pages and visits, you simply need to derive from my UserThemedPage as opposed to System.Web.UI.Page and provide a form to select the chosen theme. Trivial...You can download the component source and example from the Download Page
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