Custom style sheets can be used in flex by adding a styles.css in your configuration folder (in the same folder as functions.js). This allows you to define classes that you can reuse in multiple places or for more complex use cases, for example applying dynamic classes to elements that then apply styling to children of that element.

Note: If you are attempting to override base Flex product styles, then you may need to add !important and use high specificity on the selector in order for the styles you define to take precedence over the base product stylesheets.