MediaWiki:Vector.js

$(document).ready(function() {
    var button = document.createElement("label");
    button.innerHTML = '';
    button.id = "theme-button";
    button.role = "button";
    button.onclick = toggleTheme;

    var container = document.getElementById("p-personal");
    if (container) {
        container.parentNode.insertBefore(button, container);
    }

    var isLightTheme = localStorage.getItem("isLightTheme");
    if (isLightTheme === "true") {
        document.documentElement.classList.add('light');
    } else {
        document.documentElement.classList.remove('light');
    }
});

function toggleTheme() {
    var isLightTheme = document.documentElement.classList.toggle('light');
    localStorage.setItem("isLightTheme", isLightTheme);
}