/* ---------- Reset & biến ---------- */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --bg-primary: #0d1117;
    --bg-secondary: #161b22;
    --bg-card: #1c2333;
    --bg-input: #0d1117;
    --text-primary: #e6edf3;
    --text-secondary: #8b949e;
    --border-color: #30363d;
    --accent: #58a6ff;
    --accent-hover: #79c0ff;
    --success: #3fb950;
    --danger: #f85149;
    --radius: 10px;
    --transition: 0.25s ease;
}

body {
    font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.container {
    max-width: 820px;
    width: 100%;
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    padding: 30px 35px 20px;
    box-shadow: 0 8px 32px rgba(0, 0, 0, 0.6);
}

/* ---------- Header ---------- */
header {
    margin-bottom: 28px;
    text-align: center;
}

header h1 {
    font-size: 2rem;
    font-weight: 700;
    letter-spacing: -0.5px;
    background: linear-gradient(135deg, var(--accent), #a371f7);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.subtitle {
    color: var(--text-secondary);
    font-size: 0.95rem;
    margin-top: 4px;
}

/* ---------- Sections ---------- */
section {
    margin-bottom: 22px;
}

label {
    display: block;
    font-weight: 600;
    font-size: 0.9rem;
    color: var(--text-secondary);
    margin-bottom: 6px;
    letter-spacing: 0.3px;
}

textarea {
    width: 100%;
    padding: 14px 16px;
    background: var(--bg-input);
    color: var(--text-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    font-size: 1rem;
    font-family: 'Consolas', 'Courier New', monospace;
    resize: vertical;
    transition: border var(--transition);
    line-height: 1.6;
}

textarea:focus {
    outline: none;
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(88, 166, 255, 0.15);
}

textarea[readonly] {
    background: var(--bg-card);
    cursor: default;
}

/* ---------- Controls ---------- */
.controls {
    display: flex;
    flex-direction: column;
    gap: 18px;
    background: var(--bg-card);
    padding: 18px 20px;
    border-radius: var(--radius);
    border: 1px solid var(--border-color);
}

.control-group {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    gap: 12px;
}

.control-group label {
    margin-bottom: 0;
    min-width: 110px;
    font-weight: 500;
}

.shift-control {
    display: flex;
    align-items: center;
    gap: 14px;
    flex: 1;
    min-width: 180px;
}

.shift-control input[type="range"] {
    flex: 1;
    -webkit-appearance: none;
    appearance: none;
    height: 6px;
    background: var(--border-color);
    border-radius: 4px;
    outline: none;
    transition: background var(--transition);
}

.shift-control input[type="range"]::-webkit-slider-thumb {
    -webkit-appearance: none;
    appearance: none;
    width: 18px;
    height: 18px;
    background: var(--accent);
    border-radius: 50%;
    cursor: pointer;
    transition: 0.15s;
}

.shift-control input[type="range"]::-webkit-slider-thumb:hover {
    transform: scale(1.15);
    background: var(--accent-hover);
}

.shift-control input[type="range"]::-moz-range-thumb {
    width: 18px;
    height: 18px;
    background: var(--accent);
    border: none;
    border-radius: 50%;
    cursor: pointer;
}

.shift-control input[type="number"] {
    width: 70px;
    padding: 6px 8px;
    background: var(--bg-input);
    color: var(--text-primary);
    border: 1px solid var(--border-color);
    border-radius: 6px;
    font-size: 1rem;
    text-align: center;
    font-weight: 600;
}

.shift-control input[type="number"]:focus {
    outline: none;
    border-color: var(--accent);
}

/* ---------- Buttons ---------- */
.action-group {
    justify-content: center;
    gap: 10px;
    flex-wrap: wrap;
}

.btn {
    padding: 10px 22px;
    border: none;
    border-radius: 8px;
    font-weight: 600;
    font-size: 0.95rem;
    cursor: pointer;
    transition: all var(--transition);
    background: transparent;
    color: var(--text-primary);
    border: 1px solid var(--border-color);
}

.btn.primary {
    background: var(--accent);
    color: #0d1117;
    border-color: var(--accent);
}

.btn.primary:hover {
    background: var(--accent-hover);
    border-color: var(--accent-hover);
    transform: translateY(-1px);
    box-shadow: 0 4px 14px rgba(88, 166, 255, 0.3);
}

.btn.secondary {
    background: var(--success);
    color: #0d1117;
    border-color: var(--success);
}

.btn.secondary:hover {
    background: #4cda64;
    border-color: #4cda64;
    transform: translateY(-1px);
    box-shadow: 0 4px 14px rgba(63, 185, 80, 0.3);
}

.btn.outline {
    background: transparent;
    border: 1px solid var(--border-color);
}

.btn.outline:hover {
    background: var(--bg-card);
    border-color: var(--text-secondary);
}

.btn:active {
    transform: scale(0.96);
}

/* ---------- Copy button ---------- */
.output-section {
    position: relative;
}

.copy-btn {
    margin-top: 10px;
    width: 100%;
    padding: 10px;
    font-size: 0.9rem;
}

/* ---------- Info box ---------- */
.info-box {
    background: var(--bg-card);
    padding: 14px 18px;
    border-radius: var(--radius);
    border-left: 4px solid var(--accent);
    font-size: 0.9rem;
    color: var(--text-secondary);
    line-height: 1.6;
}

.info-box strong {
    color: var(--text-primary);
}

/* ---------- Footer ---------- */
footer {
    text-align: center;
    margin-top: 24px;
    padding-top: 16px;
    border-top: 1px solid var(--border-color);
    color: var(--text-secondary);
    font-size: 0.8rem;
}

/* ---------- Responsive ---------- */
@media (max-width: 600px) {
    .container {
        padding: 18px 16px;
    }

    header h1 {
        font-size: 1.5rem;
    }

    .control-group {
        flex-direction: column;
        align-items: stretch;
    }

    .control-group label {
        min-width: unset;
    }

    .shift-control {
        flex-wrap: wrap;
    }

    .action-group {
        flex-direction: row;
    }

    .btn {
        flex: 1;
        text-align: center;
        padding: 10px 12px;
        font-size: 0.85rem;
    }
}