istio.io/src/js/styleSwitcher.js

"use strict";

function applyStyleSheet(title) {
    let i, a;
    for (i = 0; (a = document.getElementsByTagName("link")[i]); i++) {
        if (a.getAttribute("rel").indexOf("style") !== -1 && a.getAttribute("title")) {
            a.disabled = a.getAttribute("title") !== title;
        }
    }

    // set the active theme menu item

    let item = document.getElementById("light-theme-item");
    if (item !== null) {
        if (title === "light") {
            item.classList.add("active");
        } else {
            item.classList.remove("active");
        }
    }

    item = document.getElementById("dark-theme-item");
    if (item !== null) {
        if (title === "dark") {
            item.classList.add("active");
        } else {
            item.classList.remove("active");
        }
    }
}

function getPreferredStyleSheet() {
    let i, a;
    for (i = 0; (a = document.getElementsByTagName("link")[i]); i++) {
        if (a.getAttribute("rel").indexOf("style") !== -1
            && a.getAttribute("rel").indexOf("alt") === -1
            && a.getAttribute("title")) {
            return a.getAttribute("title");
        }
    }
    return null;
}

function createCookie(name, value, days) {
    let expires = "";
    if (days) {
        const date = new Date();
        date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
        expires = "; expires=" + date.toGMTString();
    }
    document.cookie = name + "=" + value + expires + "; path=/";
}

function readCookie(name) {
    const nameEQ = name + "=";
    const ca = document.cookie.split(';');
    for (let i = 0; i < ca.length; i++) {
        let c = ca[i];
        while (c.charAt(0) === ' ') {
            c = c.substring(1, c.length);
        }

        if (c.indexOf(nameEQ) === 0) {
            return c.substring(nameEQ.length, c.length);
        }
    }
    return null;
}

function setActiveStyleSheet(title) {
    applyStyleSheet(title);
    createCookie("style", title);
}

function loadActiveStyleSheet() {
    let cookie = readCookie("style");
    if (cookie === null) {
        applyStyleSheet(getPreferredStyleSheet());
    } else {
        applyStyleSheet(cookie);
    }
}

window.onload = function (e) {
    loadActiveStyleSheet();
};

loadActiveStyleSheet();