UX polish: highlight drag-drop even when placeholder isn't visible

Before, if you scrolled so that the bottom placeholder wasn't visible, there was no indication that the app is still a drag target.

renderer/index.css

@@ -142,7 +142,6 @@ i:not(.disabled):hover {
  */
 
 .header {
-  background: rgb(40, 40, 40);
   border-bottom: 1px solid rgb(20, 20, 20);
   height: 37px; /* vertically center OS menu buttons (OS X) */
   padding-top: 6px;
@@ -159,10 +158,6 @@ i:not(.disabled):hover {
   z-index: 1;
 }
 
-.app:not(.is-focused) .header {
-  background: rgb(50, 50, 50);
-}
-
 .app.view-player .header {
   opacity: 0.8;
 }
@@ -476,6 +471,17 @@ body.drag .torrent-placeholder span {
   color: #def;
 }
 
+body.drag .app::after {
+  content: ' ';
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  background-color: rgba(0, 0, 0, 0.5);
+  z-index: 3;
+}
+
 /*
  * TORRENT LIST: EXPANDED TORRENT DETAILS
  */