add gray out of used buttons

src/roast.css

@@ -96,6 +96,13 @@ body {
     margin: 20px;
 }
 
+#event-buttons button.activated {
+    background-color: #ccc; /* gray to indicate completed step */
+    color: #333;
+    border: 2px solid #888;
+}
+
+
 #event-buttons button {
     padding: 15px 25px;
     font-size: 20px;