body { font-family: sans-serif; padding: 1em; }
h2 { margin-bottom: 1em; }
input, button { font-size: 1em; padding: 0.5em; margin: 0.5em; }
.popup { position: fixed; top: 30%; left: 10%; right: 10%; background: white; border: 2px solid black; padding: 1em; text-align: center; box-shadow: 0 4px 10px rgba(0,0,0,0.3); }
.hidden { display: none; }
button { cursor: pointer; }
.nav-buttons { margin-top: 2em; }