Add custom class example in demo

demo/electron-tabs.html

@@ -8,10 +8,15 @@
 
 <tab-group new-tab-button="true" sortable="true">
   <style>
-    /* Add custom style */
+    /* Add custom styles */
     .my-badge {
       background-color: #327BB1;
     }
+    .my-custom-tab {
+      color: red;
+      font-style: italic;
+      font-weight: bold;
+    }
   </style>
 </tab-group>
 
@@ -50,6 +55,14 @@
       tab.flash();
     }
   });
+
+  tabGroup.addTab({
+    title: "My Custom Class",
+    src: "page.html",
+    ready: function(tab) {
+      tab.element.classList.add("my-custom-tab");
+    }
+  });
 </script>
 </body>
 </html>