Thanks for writing in! Please note that the Flexbox controls are only available in Header and Footer Builder in Pro. Pro content builder or Cornerstone sections, rows, and columns are still using the conventional CSS properties. However if you need, you can always write a custom CSS class that applies a Flexbox properties to your sections, rows, and columns.
You can follow a similar resource (https://www.quackit.com/css/flexbox/examples/) and implement your custom CSS rules accordingly.
We have added this to our feature request list, so this might get added in the future.