Nested at-rules do not work

[menelaos]

Steps to reproduce:

1. stack new myapp yesodweb/simple
2. Open templates/default-layout.lucius and add e.g. a nested @media rule to the first declaration block:

   .masthead,
   .navbar {
       background-color: rgb(27, 28, 29);
   
       @media (width >= 500px) {
           background-color: red;
       }
   }

3. Start the development server
4. Observe that the background color does not change below/above 500px.

The generated CSS: