How to resize headder to fit window size?

Hi

I am using the template Hemingway and I would like to know how to customize the cover photo AND the navigation bar below it to fit any window size. I can configure the css to fit a certain amount of px but when I browse on a smaller computer the size is off. I know normally background-size: cover or simple js code will do this but it doesn’t seem to work on WordPress or there’s no where to implement javascript to my knowledge.

Please help!!