theme.js

document.body.dataset.theme = getCookie("theme");

// https://stackoverflow.com/questions/10730362/get-cookie-by-name
function getCookie(name) {
    const value = `; ${document.cookie}`;
    const parts = value.split(`; ${name}=`);
    if (parts.length === 2)
        return parts.pop().split(';').shift();
}