Increase the visibility of 404 error message (#11108)

Changed the font-size and color of the h2 element of the 404 error page so that it can look more vibrant and stand out.

css/404.scss

     text-align: center;
 
     h2 {
-        font-size: 36px;
+        font-size: 48px;
+        color : #f2f2f2;
     }
 
     &__message {
         font-size: 24px;
         margin-top: 20px;
     }
-}
+}