Update styling for app folders on the grid

Currently app folders on the grid have an almost-invisible dark gray background, and a border radius that feels small compared to some of the other UI. I had a quick go at switching both to the values used for the dash, and I think this would work:

This is the CSS I was using (literally copied over from _dash.scss):

 /* App Folders */
 .app-well-app.app-folder {
-  background-color: transparentize($osd_bg_color, 0.8);
-  border-radius: $base_border_radius + 4px; // same as %icon_tile
+  background-color: #3b3b3b;
+  border-radius: $modal_radius * 1.5;
 }

There's a glitch on hover, where the hover state still has the old border radius.

cc @jimmac @verdre