* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    -webkit-tap-highlight-color: transparent;
    user-select: none;
}

:root {
    --bg-color: #1a1a1a;
    --calculator-bg: #2d2d2d;
    --display-bg: #1e1e1e;
    --button-bg: #4a2c2a;
    --button-hover: #5d3a37;
    --operator-bg: #6b4c3c;
    --operator-hover: #8b6b5a;
    --text-color: #ffffff;
    --display-text: #d4b483;
    --shadow-color: rgba(0, 0, 0, 0.3);
    --operator-color: #ffffff;
    --equals-bg: #ffffff;
    --equals-hover: #ffffff;
    --error-color: #cf6679;
}

@media (prefers-color-scheme: light) {
    :root {
        --bg-color: #f5f5f5;
        --calculator-bg: #ffffff;
        --display-bg: #f8f9fa;
        --button-bg: #f8f9fa;
        --button-hover: #e9ecef;
        --operator-bg: #e8d5c4;
        --operator-hover: #d7c4b3;
        --text-color: #5d4037;
        --operator-color: #5d4037;
        --equals-bg: #8d6e63;
        --equals-hover: #795548;
        --error-color: #d32f2f;
    }
}

body {
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: var(--bg-color);
    padding: 0;
    margin: 0;
    transition: background-color 0.3s ease;
}

.calculator {
    background: var(--calculator-bg);
    width: 100%;
    height: 100vh;
    padding: 20px;
    display: flex;
    flex-direction: column;
    box-shadow: none;
    border-radius: 0;
    transition: background-color 0.3s ease;
}

.calculator-display {
    background-color: var(--display-bg);
    padding: 1.2rem;
    border-radius: 10px;
    margin-bottom: 1rem;
    text-align: right;
    box-shadow: inset 0 2px 5px var(--shadow-color);
    height: 25vh;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
}

.previous-operand {
    color: var(--display-text);
    font-size: 1.2rem;
    min-height: 1.5rem;
    margin-bottom: 0.5rem;
    opacity: 0.8;
}

.current-operand {
    color: var(--display-text);
    font-size: 2.5rem;
    font-weight: 500;
    word-wrap: break-word;
    word-break: break-all;
}

.calculator-buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 8px;
    height: 75vh;
    padding: 5px;
}

button {
    background-color: var(--button-bg);
    color: var(--text-color);
    border: none;
    padding: 1rem;
    font-size: 1.2rem;
    border-radius: 10px;
    cursor: pointer;
    transition: all 0.2s ease;
    box-shadow: 0 2px 5px var(--shadow-color);
    min-height: 50px;
}

button:hover {
    background-color: var(--button-hover);
    transform: translateY(-2px);
    box-shadow: 0 4px 8px var(--shadow-color);
}

button:active {
    transform: translateY(0);
    box-shadow: 0 2px 3px var(--shadow-color);
}

.operator {
    background-color: var(--operator-bg);
    color: var(--text-color);
}

.operator:hover {
    background-color: var(--operator-hover);
}

.equals {
    background: var(--equals-bg);
    grid-column: span 1;
    font-weight: 600;
    color: var(--text-color);
}

.equals:active {
    background: var(--equals-hover);
}

/* Special buttons styling */
button[data-action="clear"],
button[data-action="delete"],
button[data-action="percent"] {
    background: var(--operator-bg);
    color: var(--operator-color);
}

button[data-action="clear"]:active,
button[data-action="delete"]:active,
button[data-action="percent"]:active {
    background: var(--operator-hover);
}

/* Message Box Styles */
.message-box {
    position: fixed;
    top: 20px;
    left: 50%;
    transform: translateX(-50%);
    background: var(--calculator-bg);
    padding: 15px 25px;
    border-radius: 10px;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
    display: none;
    z-index: 1000;
    min-width: 300px;
    max-width: 90%;
    transition: background-color 0.3s ease;
}

.message-box.show {
    display: block;
    animation: slideDown 0.3s ease-out;
}

.message-content {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 15px;
}

#messageText {
    color: var(--error-color);
    font-size: 1.1rem;
    font-weight: 500;
}

.close-message {
    background: none;
    border: none;
    color: var(--text-color);
    opacity: 0.7;
    font-size: 1.5rem;
    cursor: pointer;
    padding: 0;
    width: 24px;
    height: 24px;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 50%;
    transition: all 0.2s ease;
}

.close-message:hover {
    opacity: 1;
}

@keyframes slideDown {
    from {
        transform: translate(-50%, -100%);
        opacity: 0;
    }
    to {
        transform: translate(-50%, 0);
        opacity: 1;
    }
}

/* Mobile-first responsive design */
@media (min-width: 768px) {
    .calculator {
        width: 400px;
        height: auto;
        border-radius: 20px;
        box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
        margin: 20px;
        padding: 20px;
    }

    .calculator-display {
        min-height: 120px;
        padding: 1.5rem;
    }

    .current-operand {
        font-size: 3rem;
    }

    .calculator-buttons {
        gap: 12px;
        padding: 10px;
    }

    button {
        padding: 1rem;
        font-size: 1.4rem;
        min-height: 60px;
    }
}

/* Small mobile devices */
@media (max-width: 360px) {
    .calculator {
        padding: 10px;
    }

    .calculator-display {
        padding: 1rem;
        margin-bottom: 0.8rem;
    }

    .current-operand {
        font-size: 2rem;
    }

    .calculator-buttons {
        gap: 6px;
        padding: 3px;
    }

    button {
        padding: 0.6rem;
        font-size: 1.1rem;
        min-height: 40px;
    }
}

/* Landscape mode */
@media (max-height: 500px) and (orientation: landscape) {
    .calculator {
        height: 100vh;
        padding: 10px;
    }

    .calculator-display {
        padding: 0.8rem;
        margin-bottom: 0.5rem;
    }

    .calculator-buttons {
        gap: 6px;
        padding: 3px;
    }

    button {
        padding: 0.5rem;
        font-size: 1rem;
        min-height: 35px;
    }
}