While you’re in the My Forms section, click on one of your forms and go to its Settings → Themes section. Navigate through the available themes and choose one. Press Duplicate or tweak your theme immediately.
After you’ve edited or created a theme, scroll down and press Save & Apply to Other Form if you want to apply the exact same theme to another form. In the popup window select the form you want the theme to apply to in the dropdown list and hit Apply.