:root {
    --bg: #f5f7ff;
    --card: #ffffff;
    --accent: #0f766e;
    --accent-hover: #115e59;
    --text: #0f172a;
    --subtle: #475569;
    --error: #b91c1c;
}

* {
    box-sizing: border-box;
}

body {
    margin: 0;
    font-family: Segoe UI, Tahoma, sans-serif;
    color: var(--text);
    background: radial-gradient(circle at top, #e0f2fe 0%, var(--bg) 45%);
    min-height: 100vh;
    display: grid;
    place-items: center;
    padding: 1rem;
}

.calculator-card {
    width: min(420px, 100%);
    background: var(--card);
    border-radius: 16px;
    padding: 1.5rem;
    box-shadow: 0 20px 45px rgba(15, 23, 42, 0.1);
}

h1 {
    margin: 0;
    font-size: 1.7rem;
}

.subtitle {
    margin: 0.3rem 0 1.2rem;
    color: var(--subtle);
}

.calculator-form {
    display: grid;
    gap: 0.6rem;
}

label {
    font-weight: 600;
}

input,
select,
button {
    width: 100%;
    border-radius: 10px;
    border: 1px solid #cbd5e1;
    padding: 0.65rem 0.75rem;
    font-size: 1rem;
}

input:focus,
select:focus {
    outline: 2px solid #67e8f9;
    border-color: #06b6d4;
}

button {
    margin-top: 0.4rem;
    border: none;
    background: var(--accent);
    color: #ffffff;
    font-weight: 700;
    cursor: pointer;
    transition: background 0.2s ease;
}

button:hover {
    background: var(--accent-hover);
}

.result {
    margin-top: 1rem;
    color: #166534;
}

.error {
    margin-top: 1rem;
    color: var(--error);
}