make the window thumbnails bigger

Adjust the layout of the overview and window thumbnails to make them
bigger. Also, make the background shade darker to compensate for the
increased thumbnail density.

https://bugzilla.gnome.org/show_bug.cgi?id=689876

data/theme/gnome-shell.css

@@ -613,7 +613,7 @@ StScrollBar StButton#vhandle:active {
 /* Overview */
 
 #overview {
-    spacing: 40px;
+    spacing: 24px;
 }
 
 #overview-group {
@@ -706,8 +706,8 @@ StScrollBar StButton#vhandle:active {
 }
 
 .window-picker {
-    -horizontal-spacing: 40px;
-    -vertical-spacing: 40px;
+    -horizontal-spacing: 32px;
+    -vertical-spacing: 32px;
 }
 
 /* Dash */