There is not a feature out of the box to do this. However, you can use the CSS editor to customize the appearance of the Restarter theme. It works by allowing you to add your own CSS styles, which will enable you to override the default styles of the theme.
To access the CSS editor, first launch your site customizer then click on the Additional CSS tab to access the CSS editor.
Meanwhile, I have tagged a feature request to your topic, so other people who might be interested in having this functionality on Restarter can vote for it.