JSFiddle - React, Tailwind, and code Playground

HTML

<select id="ThemeSelect" onchange="saveTheme(this.value);">
    <option value="0">Zelda</option>
    <option value="1">SMB 2</option>
</select>

<button onclick="deleteCookie('theme');">Delete</button>

JavaScript

var saveclass = null;
console.log("Initial cookie: " + document.cookie);

function saveTheme(cookieValue)
{
    var sel = document.getElementById('ThemeSelect');
    
    saveclass = saveclass ? saveclass : document.body.className;
    document.body.className = saveclass + ' ' + sel.options[sel.value].innerHTML.replace(" ", "").toLowerCase();

    setCookie('theme', cookieValue, 365);
}

function deleteCookie(name) {
    var cookie_date = new Date();  // current date & time
    cookie_date.setTime(cookie_date.getTime() - 1);
    document.cookie = name + "=; expires=" + cookie_date.toGMTString();
    console.log("After deletion: " + document.cookie);
}

function setCookie(cookieName, cookieValue, nDays) {
    var today = new Date();
    var expire = new Date();

    if (nDays==null || nDays==0)
        nDays=1;

    expire.setTime(today.getTime() + 3600000*24*nDays);
    document.cookie = cookieName+"="+escape(cookieValue) + ";expires="+expire.toGMTString();
    console.log("After change: " + document.cookie);
}