The alternate sticky header is a customization that our Themes Support team can handle for you as it is supported by our design policy. That is why you can see “15 mins” at the top of the document, which indicates how much time will be used of the 60 minutes free design time available for Shopify free themes.
If you were to reach out to our Support through our help center page, our support advisors could process your request for that customization.
Do please bear in mind that by default, Dawn’s sticky header setting is a partial sticky header, which is a persistent header that displays only when a customer scrolls up. Also, it should be avoided on stores that have all of the page content visible at all times. So it’s worth asking yourself if you need a permanently sticky header at all!