var headerBackground = true;

function updateScroll(checkHeight) {
    if (checkHeight) {
        if (headerBackground)
            document.querySelector("header").style.background = "black";
        headerBackground = false;
    } else {
        if (!headerBackground)
            document.querySelector("header").style.background = "";
        headerBackground = true;
    }
}

window.addEventListener("scroll", function() { updateScroll(window.scrollY > this.innerHeight * 0.27); });
var fixedContent = document.querySelector(".body.body-fixed");
if (fixedContent)
    fixedContent.addEventListener("scroll", function() { updateScroll(this.scrollTop > this.clientHeight * 0.27); });