black lines above and below featured image (page hero)

Hi there,

I’m using the Elements feature to apply a page hero image at the top of every page. The image is chosen using Dynamic Data, choosing the ‘featured image’ from each page.

On the home page, the image is thin and wide and is set to use ‘cover’, but there are black bands above and below the image as the image adjusts to the browser size. I can’t find the setting in the customizer to adjust this background color. How can I do this?

or is there a better way?

Thank you in advance