custom widget area not to stick to navigation menu

Dear friends,
I created a custom widget area in the header section in wordpress twenty-seventeen theme. I used the common way with putting code into the functions.php and header.php
Now, what I’d like to change: the widget is sticking to the navigation menu that is moving upwards when scrolling. I want the widget/countdown to stick to the background-image, so the navigation-menu let the countdown disappear when scrolling down.
What to do?

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