Hi Rupert,
Excellent question! You cannot create new "Preset Designs", but you can create your own Styles which do the same thing!
Here's a quick how-to on styles:
https://share.smartbuilder.com/public/support/styles.mp4We use buttons in the video, but the same holds true for any object (shapes, text, radio buttons, dropdown lists, timers, number input, etc...)
Hope that helps!