To choose a theme for your Stitch website, simply:
Click the "Customize Design" link under Stitch secondary navigation panel in your Zibbet admin
You'll arrive on the page shown above
Click the themes on the left to see a preview on the right
You can click around to explore the theme on the right-hand side
Once you've chosen a theme, save your changes
You can also click the device icons at the top-right of the left hand panel to see what the theme looks like on desktop, tablet and mobile.