Yes, you can customize the Filled and Outlined buttons in Flows using CSS customization of tooltips. The customization changes that you make at the Dashboard level reflect at the step level for all Flows while creating them using Studio.
However, the CSS customization option is not available for Custom buttons, and hence use the option Styling to style your Custom buttons. The style options you configure apply only to the specific Flow step and not the entire Flow. For more information, see Understand Action Buttons in Flows.