OK, we already have an option “Customizer – Header Options – Remove Header Gradient” … but to modify the gradient instead, use this CSS …
@media screen and (min-width: 48em) {
.twentyseventeen-front-page.has-header-image .custom-header-media:before, .twentyseventeen-front-page.has-header-video .custom-header-media:before, .home.blog.has-header-image .custom-header-media:before, .home.blog.has-header-video .custom-header-media:before {
height: 33%;
}
}
.custom-header-media:before {
background: linear-gradient(to bottom, rgba(0, 0, 0, 0) 0%, rgba(0, 0, 0, 0.3) 75%, rgba(0, 0, 0, 0.3) 100%);
}
Changing the percentage of the height statement and changing the RGBA values of the background statement will modify the gradient.
You can also change the percentage amounts in the background statement which will change where in the gradient the new RGBA values are applied. You can use this website to help you find the require RGBA values.
Let me know if this works for you and we will add a couple of options for this.
Oliver