Setka Editor and Style Manager maintain all common approaches on creating responsive web design.
You can set different paragraph and heading font properties for desktop, tablet and mobile versions of your website. In Text Formats select Screen Size > Desktop, Tablet or Mobile.
Properties for change are:
You can set custom appearance for Bold and Italic text in the same tab.
Preview area on the right shows how your post will look on different devices. Save your style changes and proceed to work with Setka Post Editor.