* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    user-select: none;
}

body {
    background-color: #0a0a0a;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    padding: 12px;
}

.calculator {
    background-color: #1c1c1e;
    border-radius: 28px;
    padding: 24px 20px 28px;
    max-width: 500px;
    width: 100%;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.8);
}

.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 16px;
    flex-wrap: wrap;
    gap: 8px;
}

.brand h1 {
    font-size: 1.2rem;
    color: #ffffff;
    letter-spacing: 1px;
}

.power-control {
    display: flex;
    align-items: center;
    gap: 8px;
}

.power-btn {
    background: none;
    border: 1px solid #555;
    color: #aaa;
    font-size: 1.5rem;
    width: 40px;
    height: 40px;
    border-radius: 50%;
    cursor: pointer;
    transition: 0.2s;
    display: flex;
    align-items: center;
    justify-content: center;
}

.power-btn:hover {
    background-color: #333;
    border-color: #888;
}

.power-btn.on {
    color: #4caf50;
    border-color: #4caf50;
    box-shadow: 0 0 8px #4caf5066;
}

.power-btn.off {
    color: #888;
    border-color: #444;
}

.power-btn:active {
    transform: scale(0.92);
}

select {
    background-color: #2c2c2e;
    border: 1px solid #3a3a3c;
    border-radius: 6px;
    color: #f0f0f0;
    padding: 4px 8px;
    font-size: 0.8rem;
    cursor: pointer;
    outline: none;
}

select:focus {
    border-color: #bb86fc;
}

/* Màn hình */
.screen {
    background-color: #0f0f11;
    border-radius: 16px;
    padding: 16px 20px;
    margin-bottom: 20px;
    min-height: 100px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    align-items: flex-end;
    word-break: break-all;
    border: 1px solid #2a2a2c;
    transition: opacity 0.3s;
}

.screen.powered-off {
    opacity: 0.2;
    pointer-events: none;
}

.expression {
    font-size: 1.1rem;
    color: #888888;
    min-height: 28px;
    width: 100%;
    text-align: right;
    font-family: 'Courier New', monospace;
}

.result {
    font-size: 2.8rem;
    font-weight: 300;
    color: #ffffff;
    width: 100%;
    text-align: right;
    font-family: 'Courier New', monospace;
    line-height: 1.2;
    transition: font-size 0.2s;
}

.result.shrink {
    font-size: 2rem;
}

.troll-note {
    font-size: 0.8rem;
    color: #ffb74d;
    margin-top: 4px;
    width: 100%;
    text-align: right;
    font-style: italic;
    min-height: 20px;
}

/* Bàn phím số - 4 cột */
.keypad {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    margin-bottom: 10px;
}

.keypad button {
    background-color: #2c2c2e;
    border: none;
    border-radius: 12px;
    color: #ffffff;
    font-size: 1.5rem;
    font-weight: 400;
    padding: 16px 0;
    cursor: pointer;
    transition: background-color 0.15s, transform 0.1s;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    box-shadow: 0 2px 4px rgba(0,0,0,0.4);
}

.keypad button:active {
    transform: scale(0.92);
    background-color: #3a3a3c;
}

/* Hàng chức năng - 4 cột */
.function-row {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    margin-bottom: 10px;
}

.func-btn {
    background-color: #2c2c2e;
    border: none;
    border-radius: 12px;
    color: #ffffff;
    font-size: 1.3rem;
    padding: 14px 0;
    cursor: pointer;
    transition: background-color 0.15s, transform 0.1s;
    font-weight: 500;
    box-shadow: 0 2px 4px rgba(0,0,0,0.4);
}

.func-btn:active {
    transform: scale(0.92);
    background-color: #3a3a3c;
}

.func-btn.operator {
    background-color: #ff9f0a;
    color: #1c1c1e;
}

.func-btn.operator:active {
    background-color: #e08e00;
}

.func-btn.equals {
    background-color: #bb86fc;
    color: #1c1c1e;
    font-weight: bold;
}

.func-btn.equals:active {
    background-color: #9f6fe6;
}

/* Khi tắt máy, vô hiệu hóa tất cả nút (trừ nút nguồn) */
.powered-off .keypad button,
.powered-off .function-row button:not(.power-btn) {
    opacity: 0.4;
    pointer-events: none;
}

/* Responsive nhỏ */
@media (max-width: 500px) {
    .calculator {
        padding: 16px 12px 20px;
        border-radius: 20px;
        max-width: 100%;
    }
    .keypad button {
        font-size: 1.2rem;
        padding: 14px 0;
    }
    .func-btn {
        font-size: 1.1rem;
        padding: 12px 0;
    }
    .power-btn {
        width: 32px;
        height: 32px;
        font-size: 1.2rem;
    }
    select {
        font-size: 0.7rem;
        padding: 2px 6px;
    }
}