image background is just like how you would in css, url(imagelink)

and the header is a transparent linear gradient, do note that the gradient will be on other elements with the primary colour, it's not that bothersome on my theme because of how dark everything else is but on lighter themes it'll probably be more noticeable linear-gradient(colour, transparent)

you could also just make the header transparent with rgba(0, 0, 0, 0) ⠀ i hope that by answering this question you will be able to come up with some cool themes ^_^