*{margin:0;padding:0;box-sizing:border-box}.app{display:flex;justify-content:center;flex-direction:column;align-items:center;text-align:center;height:100vh;transition:all .5s}.app.light{background-color:#f0f0f0;color:#333}.app.dark{background-color:#333;color:#f0f0f0}button{padding:10px 20px;border:none;cursor:pointer;margin-top:20px;font-size:16px;border:1px solid #333}