You can create a simple button for any link on the principal navigation. All you need to do is add the class name btn to your menu item and it will create a simple button. To change the color of the button, size, padding, etc, that would need to be done via custom css as I didn’t want to bloat the theme with even more options for something so simple.
Note: Don’t See the CSS Classes option? If you don’t see the field to enter your custom CSS class simply click on “Screen Options” on the top right of your menu dashboard to enable it.