* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --bg-gradient-1: #667eea;
    --bg-gradient-2: #764ba2;
    --container-bg: rgba(255, 255, 255, 0.95);
    --text-primary: #333;
    --text-secondary: #666;
    --display-bg: #f0f0f0;
    --button-bg: white;
    --button-text: #333;
    --button-shadow: rgba(0, 0, 0, 0.1);
    --mode-bg: #f5f5f5;
    --history-bg: #f8f9fa;
    --border-color: #e0e0e0;
    --number-bg: #f8f9fa;
    --scrollbar-bg: #f1f1f1;
    --scrollbar-thumb: #888;
}

body.dark-theme {
    --bg-gradient-1: #1a1a2e;
    --bg-gradient-2: #16213e;
    --container-bg: rgba(30, 30, 30, 0.95);
    --text-primary: #e0e0e0;
    --text-secondary: #b0b0b0;
    --display-bg: #2a2a2a;
    --button-bg: #3a3a3a;
    --button-text: #e0e0e0;
    --button-shadow: rgba(0, 0, 0, 0.3);
    --mode-bg: #2a2a2a;
    --history-bg: #2a2a2a;
    --border-color: #404040;
    --number-bg: #353535;
    --scrollbar-bg: #2a2a2a;
    --scrollbar-thumb: #555;
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, var(--bg-gradient-1) 0%, var(--bg-gradient-2) 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
    transition: all 0.3s ease;
}

.calculator-container {
    background: var(--container-bg);
    border-radius: 20px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3);
    padding: 30px;
    max-width: 600px;
    width: 100%;
    backdrop-filter: blur(10px);
    animation: slideIn 0.5s ease-out;
}

@keyframes slideIn {
    from {
        opacity: 0;
        transform: translateY(-20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
}

h1 {
    color: var(--text-primary);
    font-size: 28px;
    background: linear-gradient(135deg, var(--bg-gradient-1), var(--bg-gradient-2));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.theme-toggle {
    background: var(--button-bg);
    border: none;
    border-radius: 50%;
    width: 45px;
    height: 45px;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 20px;
    box-shadow: 0 3px 10px var(--button-shadow);
    transition: all 0.3s;
}

.theme-toggle:hover {
    transform: scale(1.1);
}

.display {
    background: var(--display-bg);
    border-radius: 10px;
    padding: 20px;
    margin-bottom: 20px;
    box-shadow: inset 0 2px 5px rgba(0, 0, 0, 0.1);
}

#expression {
    font-size: 18px;
    color: var(--text-secondary);
    min-height: 25px;
    word-wrap: break-word;
    text-align: right;
}

#result {
    font-size: 32px;
    font-weight: bold;
    color: var(--text-primary);
    text-align: right;
    margin-top: 10px;
    min-height: 40px;
}

.mode-selector {
    display: flex;
    gap: 10px;
    margin-bottom: 20px;
    background: var(--mode-bg);
    border-radius: 10px;
    padding: 5px;
}

.mode-btn {
    flex: 1;
    padding: 10px;
    border: none;
    background: transparent;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.3s;
    font-weight: 500;
    color: var(--text-secondary);
}

.mode-btn.active {
    background: var(--button-bg);
    color: var(--bg-gradient-1);
    box-shadow: 0 2px 10px rgba(102, 126, 234, 0.2);
}

.calculator-section, .converter-section {
    display: none;
}

.calculator-section.active, .converter-section.active {
    display: block;
}

.converter-grid {
    display: grid;
    gap: 20px;
}

.converter-card {
    background: var(--display-bg);
    border-radius: 10px;
    padding: 20px;
}

.converter-card h3 {
    color: var(--text-primary);
    margin-bottom: 15px;
    font-size: 18px;
}

.converter-inputs {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    gap: 10px;
    align-items: center;
}

.converter-input {
    display: flex;
    flex-direction: column;
    gap: 5px;
}

.converter-input label {
    color: var(--text-secondary);
    font-size: 12px;
}

.converter-input input {
    padding: 10px;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    background: var(--button-bg);
    color: var(--text-primary);
    font-size: 16px;
}

.converter-input select {
    padding: 10px;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    background: var(--button-bg);
    color: var(--text-primary);
    font-size: 14px;
    cursor: pointer;
}

.converter-arrow {
    font-size: 20px;
    color: var(--text-secondary);
}

.buttons-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

.scientific-buttons {
    display: none;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    margin-bottom: 10px;
}

.scientific-buttons.active {
    display: grid;
}

button {
    padding: 20px;
    font-size: 18px;
    border: none;
    border-radius: 10px;
    cursor: pointer;
    background: var(--button-bg);
    color: var(--button-text);
    box-shadow: 0 3px 10px var(--button-shadow);
    transition: all 0.3s;
    font-weight: 500;
}

button:hover {
    transform: translateY(-2px);
    box-shadow: 0 5px 20px var(--button-shadow);
}

button:active {
    transform: translateY(0);
    box-shadow: 0 2px 5px var(--button-shadow);
}

.operator {
    background: linear-gradient(135deg, #667eea, #764ba2);
    color: white;
}

.equals {
    background: linear-gradient(135deg, #f093fb, #f5576c);
    color: white;
    grid-column: span 2;
}

.clear {
    background: #ff6b6b;
    color: white;
}

.number {
    background: var(--number-bg);
}

.function {
    background: linear-gradient(135deg, #4facfe, #00f2fe);
    color: white;
    font-size: 16px;
}

.zero {
    grid-column: span 2;
}

.history {
    margin-top: 20px;
    padding: 15px;
    background: var(--history-bg);
    border-radius: 10px;
    max-height: 200px;
    overflow-y: auto;
}

.history h3 {
    color: var(--text-primary);
    margin-bottom: 10px;
    font-size: 16px;
}

.history-item {
    padding: 5px;
    color: var(--text-secondary);
    font-size: 14px;
    border-bottom: 1px solid var(--border-color);
}

.history-item:last-child {
    border-bottom: none;
}

.unit-categories {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(100px, 1fr));
    gap: 10px;
    margin-bottom: 20px;
}

.unit-category-btn {
    padding: 12px;
    background: var(--button-bg);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.3s;
    color: var(--text-primary);
    font-size: 14px;
}

.unit-category-btn.active {
    background: linear-gradient(135deg, #667eea, #764ba2);
    color: white;
    border-color: transparent;
}

.unit-category-btn:hover {
    transform: translateY(-2px);
}

::-webkit-scrollbar {
    width: 6px;
}

::-webkit-scrollbar-track {
    background: var(--scrollbar-bg);
    border-radius: 10px;
}

::-webkit-scrollbar-thumb {
    background: var(--scrollbar-thumb);
    border-radius: 10px;
}

::-webkit-scrollbar-thumb:hover {
    background: #555;
}

@media (max-width: 480px) {
    button {
        padding: 15px;
        font-size: 16px;
    }
    
    #result {
        font-size: 24px;
    }

    .converter-inputs {
        grid-template-columns: 1fr;
        gap: 15px;
    }

    .converter-arrow {
        transform: rotate(90deg);
        text-align: center;
    }
}