:root {
    --bg-gradient: linear-gradient(135deg, #f1f5f9 0%, #e2e8f0 100%);
    --calc-bg: rgba(255, 255, 255, 0.8);
    --display-text: #1e293b;
    --text-secondary: #64748b;
    --btn-num: #ffffff;
    --btn-num-hover: #f8fafc;
    --btn-accent: #6366f1;
    --btn-accent-hover: #4f46e5;
    --btn-secondary: #e2e8f0;
    --shadow: 0 20px 50px rgba(0, 0, 0, 0.05);
    --border: rgba(255, 255, 255, 0.3);
}

[data-theme="dark"] {
    --bg-gradient: linear-gradient(135deg, #0f172a 0%, #1e293b 100%);
    --calc-bg: rgba(30, 41, 59, 0.7);
    --display-text: #f8f8fc;
    --text-secondary: #94a3b8;
    --btn-num: #334155;
    --btn-num-hover: #475569;
    --btn-accent: #818cf8;
    --btn-accent-hover: #6366f1;
    --btn-secondary: #1e293b;
    --shadow: 0 20px 50px rgba(0, 0, 0, 0.3);
    --border: rgba(255, 255, 255, 0.05);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Inter', sans-serif;
}

body {
    background: var(--bg-gradient);
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    transition: all 0.4s ease;
}

/* --- Calculator Container --- */
.calculator-wrapper {
    width: 360px;
    background: var(--calc-bg);
    backdrop-filter: blur(20px);
    border-radius: 32px;
    padding: 24px;
    box-shadow: var(--shadow);
    border: 1px solid var(--border);
    position: relative;
    overflow: hidden;
}

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

.calc-logo {
    display: flex;
    align-items: center;
    gap: 8px;
    font-weight: 600;
    color: var(--btn-accent);
}

.header-actions button {
    background: transparent;
    border: none;
    color: var(--text-secondary);
    font-size: 1.1rem;
    cursor: pointer;
    margin-left: 10px;
    transition: 0.3s;
}

.header-actions button:hover {
    color: var(--btn-accent);
}

/* --- Display Area --- */
.display-area {
    text-align: right;
    margin-bottom: 30px;
    padding: 10px;
}

.expression {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1rem;
    color: var(--text-secondary);
    min-height: 1.5rem;
    margin-bottom: 5px;
    word-break: break-all;
}

.result {
    font-family: 'JetBrains Mono', monospace;
    font-size: 3rem;
    font-weight: 500;
    color: var(--display-text);
    overflow: hidden;
    text-overflow: ellipsis;
}

/* --- Keypad Grid --- */
.keypad {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

.btn {
    height: 65px;
    border-radius: 18px;
    border: none;
    font-size: 1.25rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
    display: flex;
    justify-content: center;
    align-items: center;
}

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

.btn-number {
    background: var(--btn-num);
    color: var(--display-text);
}

.btn-number:hover {
    background: var(--btn-num-hover);
}

.btn-accent {
    background: rgba(99, 102, 241, 0.1);
    color: var(--btn-accent);
}

.btn-accent:hover {
    background: var(--btn-accent);
    color: white;
}

.btn-secondary {
    background: var(--btn-secondary);
    color: var(--display-text);
}

.btn-equal {
    background: var(--btn-accent);
    color: white;
    grid-column: span 1;
    box-shadow: 0 8px 15px rgba(99, 102, 241, 0.3);
}

/* --- History Panel --- */
.history-panel {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: var(--calc-bg);
    backdrop-filter: blur(30px);
    transform: translateY(100%);
    transition: transform 0.4s cubic-bezier(0.175, 0.885, 0.32, 1.275);
    padding: 24px;
    z-index: 10;
}

.history-panel.active {
    transform: translateY(0);
}

.history-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
    border-bottom: 1px solid var(--border);
    padding-bottom: 10px;
}

#history-list {
    list-style: none;
    max-height: 80%;
    overflow-y: auto;
}

.history-item {
    margin-bottom: 15px;
    text-align: right;
    animation: fadeIn 0.3s ease;
}

.history-exp {
    font-size: 0.85rem;
    color: var(--text-secondary);
}

.history-res {
    font-weight: 600;
    color: var(--btn-accent);
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateX(10px); }
    to { opacity: 1; transform: translateX(0); }
}

.footer {
    margin-top: 30px;
    color: var(--text-secondary);
    font-size: 0.8rem;
    opacity: 0.7;
}

/* --- Mobile Optimization --- */
@media (max-width: 400px) {
    .calculator-wrapper {
        width: 100%;
        height: 100vh;
        border-radius: 0;
    }
}