:root {
    --bg-color: #131419;
    --primary-glow: #9f40ff;
    --secondary-glow: #00e0ff;
    --calc-bg: rgba(35, 38, 51, 0.5);
    --border-color: rgba(255, 255, 255, 0.05);
    --shadow-color: rgba(0, 0, 0, 0.3);
    --display-bg: rgba(0, 0, 0, 0.25);
    --text-color: #fff;
    --history-color: rgba(255, 255, 255, 0.5);
    --button-bg: rgba(255, 255, 255, 0.05);
    --button-hover-bg: rgba(255, 255, 255, 0.1);
    --operator-color: var(--secondary-glow);
    --equals-bg: var(--secondary-glow);
    --equals-color: #131419;
}

body.light-theme {
    --bg-color: #f0f2f5;
    --primary-glow: #ff7f50;
    --secondary-glow: #1e90ff;
    --calc-bg: rgba(255, 255, 255, 0.5);
    --border-color: rgba(0, 0, 0, 0.05);
    --shadow-color: rgba(100, 100, 100, 0.2);
    --display-bg: rgba(255, 255, 255, 0.6);
    --text-color: #131419;
    --history-color: rgba(0, 0, 0, 0.5);
    --button-bg: rgba(0, 0, 0, 0.03);
    --button-hover-bg: rgba(0, 0, 0, 0.07);
    --operator-color: var(--secondary-glow);
    --equals-bg: var(--secondary-glow);
    --equals-color: #fff;
}

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

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: var(--bg-color);
    transition: background-color 0.4s ease;
    overflow: hidden;
    position: relative;
}

body::before, body::after {
    content: '';
    position: absolute;
    width: 400px;
    height: 400px;
    border-radius: 50%;
    filter: blur(150px);
    z-index: -1;
    transition: background-color 0.4s ease;
}

body::before {
    background-color: var(--primary-glow);
    top: 15%;
    left: 15%;
}

body::after {
    background-color: var(--secondary-glow);
    bottom: 15%;
    right: 15%;
}

.calculator {
    width: 320px;
    background: var(--calc-bg);
    backdrop-filter: blur(25px);
    border: 1px solid var(--border-color);
    border-radius: 25px;
    box-shadow: 0 25px 45px var(--shadow-color);
    padding: 20px;
    transition: background-color 0.4s ease;
}

/* --- Display --- */
.display {
    margin-bottom: 20px;
    background: var(--display-bg);
    border-radius: 15px;
    padding: 10px 15px;
    box-shadow: inset 0 5px 10px rgba(0,0,0,0.1);
}
.display-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    color: var(--history-color);
    font-size: 0.9rem;
}
.theme-toggle {
    cursor: pointer;
    transition: color 0.3s ease;
}
.theme-toggle:hover {
    color: var(--secondary-glow);
}
.memory-indicator {
    font-weight: bold;
    opacity: 0;
    transition: opacity 0.3s ease;
}
.memory-indicator.active {
    opacity: 1;
}

.display-content {
    text-align: right;
}
.history {
    height: 20px;
    font-size: 0.9rem;
    color: var(--history-color);
    overflow: hidden;
    white-space: nowrap;
}
.result {
    width: 100%;
    border: none;
    background: transparent;
    font-size: 2.5rem;
    color: var(--text-color);
    text-align: right;
    outline: none;
    font-weight: 300;
}
.result::placeholder {
    color: var(--text-color);
    opacity: 0.7;
}

/* --- Buttons --- */
.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 15px;
}
.btn {
    background: var(--button-bg);
    border: 1px solid var(--border-color);
    border-radius: 15px;
    padding: 15px;
    font-size: 1.2rem;
    color: var(--text-color);
    cursor: pointer;
    transition: all 0.2s ease;
    box-shadow: 0 5px 15px rgba(0,0,0,0.1);
}
.btn:hover {
    background: var(--button-hover-bg);
}
.btn:active {
    transform: scale(0.95);
    box-shadow: 0 2px 10px rgba(0,0,0,0.2);
}
.btn.operator {
    color: var(--operator-color);
}
.btn.equals {
    grid-column: span 1;
    background: var(--equals-bg);
    color: var(--equals-color);
    font-weight: 600;
}
.btn.equals:hover {
    filter: brightness(1.1);
}