When the first Theme is selected it automatically gives the Predefined Style for Tabs but if you want to Customize Tabs you can follow this guide.
1) Drag and Drop Tabs from left side widget section then a Dialogue Box will pop up by this action for Tabs as shown in image below .
2) Now in Settings section of Dialog Box you can open Tab and add text of your own as shown in image below.
3) In Settings section of Dialogue Box you can also select icons and customize as shown in figure below.
4) Now in Style section of Dialog Box you can give Custom Style and also use Predefined Styles as shown in image below.