In this forum we do not provide support for CSS questions only Hugo related questions.
You need to ask at the Ananke theme’s repo issues tracker about offering a parameter that controls they height of the header image.
With that said one way to go about it would be to set the div .bg-black-60 so that its height is always that of the viewport irrespective of content i.e. height: 100vh
But then again that might not be ideal and you may need to redesign parts of the theme.
Therefore it is best to ask the theme authors as they are the ones who can help you best.