You find the design settings in the right hand side of an email draft or survey draft.
You can change font, font-size, background-images, border-radius, link colors, width, margins etc.
When you’re happy with your design you can save it as a THEME. This means that if you for example have made a nice email design and saved it as a theme you then can create a survey and reuse the design by using the same theme. (Why reinvent the wheel?)