body {
    min-height: 100vh;
    background: linear-gradient(135deg, #181a1b 0%, #232526 40%, #1a2634 100%);
    background-attachment: fixed;
}
.card {
    border-radius: 18px;
    border: 4px solid rgba(40, 62, 81, 0.85); /* Borde más grueso y visible */
    box-shadow: 0 0 24px 6px rgba(40, 62, 81, 0.45), 
                0 8px 32px 0 rgba(31, 38, 135, 0.25); /* Difuminado exterior */
    backdrop-filter: blur(2px);
}
.card-header {
    border-radius: 18px 18px 0 0;
}
.card-header.bg-primary {
    background-color: #283e51 !important; /* Azul petróleo oscuro */
}
.form-control {
    border-radius: 8px;
    border: none;
}
.form-control.bg-secondary {
    background-color: #3a3f47 !important; /* Gris más claro y visible */
    color: #fff !important;
    border: 1.5px solid #6c757d;
}
.form-control:focus {
    background-color: #464b53 !important;
    color: #fff !important;
    border-color: #b0b7c3;
    box-shadow: 0 0 0 2px #283e5133;
}
#resultado {
    border-radius: 8px;
}