Added missing NOTIFICATION_DARK (#11770)

And corrected couple of typos

Addresses https://github.com/vaadin/framework/issues/11492

server/src/main/java/com/vaadin/ui/themes/ValoTheme.java

@@ -100,13 +100,13 @@ public class ValoTheme {
 
     /**
      * Success notification style. Adds a border around the notification and an
-     * icon next to the title. Can be combined with any other Label style.
+     * icon next to the title. Can be combined with any other Notification style.
      */
     public static final String NOTIFICATION_SUCCESS = "success";
 
     /**
      * Failure notification style. Adds a border around the notification and an
-     * icon next to the title. Can be combined with any other Label style.
+     * icon next to the title. Can be combined with any other Notification style.
      */
     public static final String NOTIFICATION_FAILURE = "failure";
 
@@ -117,6 +117,12 @@ public class ValoTheme {
      */
     public static final String NOTIFICATION_CRITICAL_ERROR = "critical-error";
 
+    /**
+     * Styles the notification to be dark variant. Can be combined with any
+     * other Notification style.
+     */
+    public static final String NOTIFICATION_DARK = "dark";
+
     /***************************************************************************
      *
      * Label styles