Click Settings in the designer's top bar. The modal contains:

• Title – the template's name. In the current version this field is shown read-only for any template that has already been saved (not just flow templates) – you can only set it once, when the template is first created.
• Background – the page background colour behind the email (default #F5F6F6).
• Text color – default text colour (default #6F6F6F).
• Width – email body width in px (new templates use 640 px).
• Border radius – rounding of the email body in px.
• Margin top – space above the email body in px (default 50).
• Padding Left / Padding Right – inner horizontal padding in px (default 40).
• Template – shows the current layout with an Edit Template option to change layout.

Press SAVE to apply. Super-admins additionally see a Delete button here; regular users remove designs from the Templates list instead.