In the Flex theme, the Default page template includes a couple of settings in the theme editor for the layout and to enter some custom CSS.
Access Template Settings
In the theme editor (Customize), use the page selector to open Pages, then Default page.
In the left sidebar, open Page content to customize general settings for the template.
The width can either be set to Standard or Wide. Selecting Wide will spread out the full width of the screen. Selecting Standard will keep the section in a container and not spread the full width.
You can choose to add space (padding) above and below the section by adjusting the Top spacing and Bottom spacing settings.
Add Custom CSS to the feature for further tweaking. You can even create your own CSS class here, as well. Check out this article for more information: Flex Guide: Custom CSS Feature