ASP.NET MVC 6 (vNext) not properly linking to files

I'm trying to run the ASP.NET 5 MVC 6 sample "HelloMVC" project, with a custom CSS file.

I place my .css files in the wwwroot folder, let's say in a styles folder.

I then link to it in my _Layout.cshtml with

<link rel="stylesheet" href="~/styles/mystyle.css" />

However, the style doesn't get rendered.

If I go to the URL of the expected file in my browser (let's say I'm running on port 12345, so I go to http://localhost:12345/styles/mystyle.css, I'm greeted with a cheeky page (pure HTML) with a lightbulb and a smiley face that says "Your ASP.NET 5 application has been successfully started".

In fact, if I go to any URL for the running app, I'm greeted with the same page. i.e. going to http://localhost:12345/thisfiledoesntexist.jpg will return the same page.

Is this ASP.NET 5's current 404 page? Why aren't my styles getting rendered? I would think this would be a very simple thing.

Looking at this question it seems that what I'm doing would be the proper way. In fact, Visual Studio even has intellisense for the css files that I've added when typing out the href.