Guys, i really don't know what computers you all have, what browsers and screen settings.
but i double checked and the header image is a non transparent png that has a gradient at both sides that ends in the same color as the background color of the site.
From what i see in Buxee's image, the background color (the lighter one) is 44454a which is definitely not in the css of the site. the actual bg color is 1a1b1d
which is the same color as the ending of the header png.
i tried different settings of brightness, tested on 3 laptops 15", 17", 1366px, 1440px and 1920 px resolutions, as well as on a 24" 1920 px screen. i can't replicated the issue.