How to limit size of header image?

Can anyone give me an idea of how to limit the size of the header image? It automatically resizes to proportionally fit the window as the window is resized - a useful feature. But I'd like to limit the maximum expansion to the size of the source image, which happens to be 1200x250 - preferably while being centered horizontally in the available space. I tried adding this to style.css in the child theme, and saw no effect: .header-featured-image img { max-height: 250px; max-width: 1200px; align-content:center; } Advice appreciated.