switchTheme.js

function switchTheme(ev) {
    // If the alt/option key is pressed, reset to system default
    if (ev.altKey) {
        document.getElementsByTagName("BODY")[0].removeAttribute("theme");
        return; 
    }

    const prefersDark = window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches;

    let theme = document.getElementsByTagName("BODY")[0].getAttribute("theme");
    if (theme === "dark") {
        theme = "light"; 
    } else if (theme === "light") {
        theme = "dark";
    } else {
        theme = prefersDark ? "light" : "dark";
    }
    document.getElementsByTagName("BODY")[0].setAttribute("theme", theme);
}