How to add additional header below main header on Mobile?

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So say my mobile site looks like this: [https://gyazo.com/cef6b2b29e684251c6bf2346bc07a66b])

But I want to add a seconder area below the header like this: [https://gyazo.com/d2fc8a191240579ee4f13dff91f14ad2])

How would you achieve this? Seems so simple, but the theme for this site has literally no customization options, so I know I gotta code it in somehow, but where do I go?

And how do I code it so that it only shows up on mobile, nothing else?

Any suggestions or help will seriously be appreciated. Thanks everyone.

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