style.css

.has-header-image .custom-header-media img, .has-header-video .custom-header-media video, .has-header-video .custom-header-media iframe, .has-header-image:not(.twentyseventeen-front-page):not(.home) .custom-header-media img {
height: 66% !important;
object-fit: fill !important;
width: 100%;
max-height: 100% !important;
display: block !important;
position: relative !important;
}

#wp-custom-header {
height: 50% !important;
}

.admin-bar.twentyseventeen-front-page.has-header-image .custom-header-media, .admin-bar.twentyseventeen-front-page.has-header-video .custom-header-media, .admin-bar.home.blog.has-header-image .custom-header-media, .admin-bar.home.blog.has-header-video .custom-header-media {
height: calc(71vh – 32px) !important;
}

Hi!

It seems that if I insert it to the Custom CSS part your code is not working well.

I suggest the following method:

.has-header-image.twentyseventeen-front-page .custom-header, .has-header-video.twentyseventeen-front-page .custom-header, .has-header-image.home.blog .custom-header, .has-header-video.home.blog .custom-header
{
	height: 400px;
}

Works well on mobile and PC.