Slider, limit image height

I have images for my slider that are below the recommended 1800×800 size. Currently the theme appears to be stretching to the maximum width. I would like to prevent the theme from stretching the images past a certain height (say 600px). I understand that this would create a gap on both sides when the screen width is wider than the image (assuming it stays centered). Is there a simple css or js modification that would limit the slider height to a maximum number of pixels (without breaking the adaptive size at smaller widths)?

Thank you to the developers for designing and releasing this theme.