Port Lightbox to CSS and use it in runDialog

Make Lightbox stylable and add a common style. Use it in runDialog.
https://bugzilla.gnome.org/show_bug.cgi?id=604978

data/theme/gnome-shell.css

@@ -773,13 +773,13 @@ StTooltip {
     color: white;
 }
 
-.run-dialog-box {
-    background: rgba(0,0,0,0.3);
-}
-
 .run-dialog {
     padding: 8px;
     border: 1px solid rgba(128,128,128,0.40);
     border-radius: 4px;
     background: rgba(0,0,0,0.8);
 }
+
+.lightbox {
+    background-color: rgba(0, 0, 0, 0.27);
+}