Snackbar's color can be changed according to the message type

- load a separate css file to change the color
- if the assignment failed, the snackbar prompt will be red, otherwise theme color

#story[1278]

src/main/resources/css/dialogs/Snackbar.css

@@ -0,0 +1,4 @@
+/* a separate file to dynamically change snackbar's color */
+.jfx-snackbar-toast {
+    -fx-text-fill: red !important;
+}