* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
    background: #f0f2f5;
    display: flex;
    flex-direction: column;
    align-items: center;
    min-height: 100vh;
    padding: 50px 20px;
}

h1 {
    color: #333;
    margin-bottom: 20px;
}

form {
    background: #fff;
    padding: 30px 40px;
    border-radius: 12px;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.1);
    width: 320px;
    text-align: center;
}

input[type="number"] {
    width: 100%;
    padding: 12px 15px;
    margin-bottom: 20px;
    border: 1px solid #ccc;
    border-radius: 8px;
    font-size: 1em;
}

input[type="submit"] {
    background-color: #4CAF50;
    color: white;
    padding: 12px 20px;
    border: none;
    border-radius: 8px;
    font-size: 1em;
    cursor: pointer;
    transition: 0.3s;
    width: 100%;
}

input[type="submit"]:hover {
    background-color: #45a049;
}

.resultado {
    margin-top: 50px; 
    padding: 15px;
    border-radius: 8px;
    font-weight: bold;
    font-size: 1.2em;
    text-align: center;
    width: 320px;
}

.bisiesto {
    background-color: #d4edda;
    color: #155724;
    border: 1px solid #c3e6cb;
}

.no-bisiesto {
    background-color: #f8d7da;
    color: #721c24;
    border: 1px solid #f5c6cb;
}