Hi there,
the gradient is added using some CSS – if you want to remove that then go to Customizer > Additional CSS – find this:
.page-hero.gradient {
background-image: linear-gradient(145deg, transparent 0%, rgba(255, 255, 255, 0.5) 100%);
max-width: 1440px;
margin: auto;
}
And remove this line:
background-image: linear-gradient(145deg, transparent 0%, rgba(255, 255, 255, 0.5) 100%);
Then go to Appearance > Elements and look for the Header Element – where you can select a Background Image > Custom Image.