Thank you for writing in, please use the method provided here instead. They use that for making the Transparent header change to Opaque on Scroll. The same method can be used for changing the height of navbar using the min-height.
Also, the way you added that banner-image on the header is kind of hacky. You might want to follow that guide from the top instead, which use the first section as the hero image. You would not need the min-height if you follow that.
Hope it helps,