stop making navbar transparent just because we're on desktop

As far as i can tell this works in Todd's styles only because everything is given a 'light'
header; removing that class from the blog post for example gives the same problem as we see
in our code.

Partly fixes #33

agaric/sass/agaric/_component.navbar.scss

@@ -37,7 +37,6 @@
   }
   @include desktop {
     padding: 1.25em 0;
-    background-color: transparent;
   }
   z-index: +9;
 }