userscript-github-remove_dark_header.user.js

// ==UserScript==
// @name            GitHub: Remove header-dark
// @namespace       https://github.com/Zren/
// @description     Change the dark header to the old lighter one.
// @icon            https://github.com/favicon.ico
// @author          Zren
// @version         2
// @match           https://github.com/*
// @match           https://gist.github.com/*
// @grant           none
// @run-at          document-start
// ==/UserScript==

function lightHeader() {
    var e = document.querySelector('.header.header-dark');
    if (e) {
        e.classList.remove('header-dark');
    }
}
lightHeader();