body{margin:0;font-family:Arial,Helvetica,sans-serif;transition:background .3s,color .3s}body{background:#f5f5f5;color:#222}body.dark{background:#121212;color:#fff}.container{min-height:100vh;display:flex;flex-direction:column;justify-content:center;align-items:center;transition:.3s}button{padding:10px 20px;font-size:16px;cursor:pointer;border:none;border-radius:6px;margin-bottom:20px;transition:.3s}body:not(.dark) button{background:#222;color:#fff}body.dark button{background:#fff;color:#222}