.NET MVC 5 Themed Partial Extension Method

_Layout.cshtml

@using ProjectName.Extensions

@Html.ThemePartial( "WelcomeMessage" )

@html.ThemePartial( "SomeCopyText" )

SomeCopyText.cshtml

<p>Text that does not need to be themed</p>

ThemeHtmlExtensions.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.IO;
using System.Linq;
using System.Web.Hosting;
using System.Web.Mvc;
using System.Web.Mvc.Html;

namespace ProjectName.Extensions {

	/// <summary>
	///     Extends the default HtmlHelper class for theme related methods
	/// </summary>
	public static class ThemeHtmlExtensions {

		/// <summary>
		///     Renders a themed partial view, falls back to no theme if no partial exists for the theme
		/// </summary>
		/// <param name="htmlHelper"></param>
		/// <param name="partialViewName"></param>
		/// <returns></returns>
		public static MvcHtmlString ThemePartial( this HtmlHelper htmlHelper , string partialViewName ) {
			var controllerName = htmlHelper.ViewContext.RouteData.GetRequiredString( "controller" );

			var themeName = ConfigurationManager.AppSettings.Get( "Theme" ) ?? "Theme";

			var pathsToSearch = new List<string>();

			var possibleExtensions = new List<string> {
				"aspx" ,
				"ascx" ,
				"cshtml" ,
				"vbhtml" ,
			};

			foreach ( var extension in possibleExtensions ) {
				pathsToSearch.Add( string.Format( "~/Views/Shared/{0}.{1}.{2}" , partialViewName , themeName , extension ) );
				pathsToSearch.Add(
					string.Format( "~/Views/{0}/{1}.{2}.{3}" , controllerName , partialViewName , themeName , extension ) );
				pathsToSearch.Add( string.Format( "~/Views/Shared/{0}.{1}" , partialViewName , extension ) );
				pathsToSearch.Add( string.Format( "~/Views/{0}/{1}.{2}" , controllerName , partialViewName , extension ) );
			}

			// TODO: Extend for areas

			var path = pathsToSearch.FirstOrDefault( query => File.Exists( HostingEnvironment.MapPath( query ) ) );

			if ( string.IsNullOrEmpty( path ) ) {
				throw new InvalidOperationException(
					string.Format(
						"The themed partial view '{0}' was not found or no view engine supports the searched locations. The following locations were searched:{1}{2}" ,
						partialViewName ,
						Environment.NewLine ,
						string.Join( Environment.NewLine , pathsToSearch ) ) );
			}

			return htmlHelper.Partial( path );
		}

	}

}

web.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
...
	<appSettings>
		<add key="Theme" value="MyTheme" />
	</appSettings>
...
</configuration>

WelcomeMessage.cshtml

<h1>Default Welcome!</h1>

WelcomeMessage.MyTheme.cshtml

<h1>Themed Welcome!</h1>