It’s so easy to do. Just define in front matter an image and a pattern image. Instead of a pattern you can also use a color. Have a look at the example with a background color.
Front Matter Code
header: image: "header_unsplash_2-970x.jpg" pattern: "pattern_concrete.jpg" caption: This is a caption for the header image with link caption_url: https://unsplash.com/
design pattern header