The details shared here apply to the most recent version of Flex. To keep your theme up to date, check out this helpful document.
Using the Page Template
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.
When adding a Page in your Shopify Admin (Online Store > Pages), the default page template is named page
and is the one that is already selected under Template > Template Suffix:
Once you've added your page content, you can head into the Theme Editor (Customize) > Select your Page from the drop-down page selector. Your theme editor on the left will then update with the page settings. Here you can adjust the 'Layout' and 'Advanced' settings if you'd like!
Layout
In the layout settings, you can adjust the width of the page content, and the top/bottom spacing.
If you'd like your page content to spread full-width, you can select 'wide.' If you'd like your page content to be contained/not full-width, you can select 'standard.'
You can adjust the space between your page content and the header/footer with the top spacing and bottom spacing settings.
Advanced
In the advanced settings, you can add in your own CSS class and custom CSS.
More information about using the Advanced CSS feature.
In the 'CSS Class' field, you can create a new class to apply to the section, or you can add a class that already exists to apply those styles as well!
The 'Custom CSS' field is where you can add in the actual CSS code.