A style preset is a group of CSS parameters built within the themes, assigned with unique template or object name or ID, so it can be applied and reused by tiles and text sections within the site using the same theme. The feature which allows style presets to be reused gives content administrators more control in replicating branded styles within the site, and it is done through the site editor.
Different design elements of a tile and text section can be customized as you create a new style preset, or even if you just edit an existing one in the theme, such as font transparency, margin, heading style line height, and so much more!
Just be careful with editing an existing style preset, as the changes will be applied to all the tiles or text sections using the style preset, and to all the sites using the theme where that named style preset reside. If you have to edit an existing style preset, it is best to duplicate the theme first.
Adding a new style preset doesn't really affect any existing site, so there is need to duplicate the theme, for this case.