Thank you for providing the link to your site!
You can achieve what you’re after with some custom CSS.
To add custom CSS, firstly set up a child theme or activate a custom CSS plugin. If you have Jetpack installed then you can activate its custom CSS module.
Enter the following snippet in either the editor for your CSS plugin or the style.css file of your child theme:
.hero.with-featured-image {
position: relative;
}
.hero.with-featured-image:before {
content: "";
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
background-color: rgba(0,0,0,0.225);
}
@media screen and (min-width: 1020px) {
.hero-image .site-header {
background: transparent;
}
}
Let me know how you get on with that or if you have any extra questions.