Startup Pro: Nav Menu Buttons
The theme demo displays a 'Get Started' button in the header navigation menu. To add a button to your menu, navigate to Appearance → Menus and select the menu you wish to edit.
Under the Screen Options tab, make sure the CSS Classes checkbox is checked:
Scroll down to the menu item you wish to edit and expand it. In the CSS Classes field, add the word button:
You can also use a combination of other button classes. A full list of button classes included in the theme is available here https://docs.seothemes.com/article/174-startup-pro-button-classes.
The theme also provides the hide-mobile and hide-desktop classes which allow you to show different buttons at different screen sizes. By using these classes on 2 different menu items, the theme demo displays a white button on large screens and a colored button on smaller screens.