CSS tweaks to match mockups better

- add some spacing between buttons
- move controls closer to the workspaces view (we'll need that space
  for the message tray)
- fix the look of the scrollbar background
- adjust sizes of theme images
- some general CSS cleanup

https://bugzilla.gnome.org/show_bug.cgi?id=607872

js/ui/overview.js

@@ -232,7 +232,7 @@ Overview.prototype = {
         // place the 'Add Workspace' button in the bottom row of the grid
         this._workspacesBarX = this._workspacesX;
         this._workspacesBarWidth = primary.width - this._workspacesBarX - WORKSPACE_GRID_PADDING;
-        this._workspacesBarY = primary.height - displayGridRowHeight + 5;
+        this._workspacesBarY = primary.height - displayGridRowHeight;
 
         // The parent (this._group) is positioned at the top left of the primary monitor
         // while this._backOver occupies the entire screen.