:root {
    /* Light Mode Variables */
    --bg-color: #f4f4f9;
    --container-bg: #ffffff;
    --text-color: #333;
    --display-bg: #e0e0e0;
    --btn-bg: #f0f0f0;
    --btn-hover: #dcdcdc;
    --accent-color: #007bff;
    --accent-text: #fff;
    --operator-color: #ff9f43;
}

body.dark-mode {
    /* Dark Mode Variables */
    --bg-color: #1e1e2e;
    --container-bg: #2d2d44;
    --text-color: #ffffff;
    --display-bg: #181825;
    --btn-bg: #3b3b58;
    --btn-hover: #4e4e75;
    --accent-color: #89b4fa;
    --accent-text: #1e1e2e;
    --operator-color: #fab387;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    transition: background-color 0.3s, color 0.3s;
}

body {
    background-color: var(--bg-color);
    color: var(--text-color);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
}

/* Theme Toggle Button */
.theme-btn {
    position: absolute;
    top: 20px;
    right: 20px;
    padding: 10px 15px;
    background-color: var(--btn-bg);
    color: var(--text-color);
    border: 1px solid var(--text-color);
    border-radius: 20px;
    cursor: pointer;
    font-weight: bold;
}

.container {
    background-color: var(--container-bg);
    padding: 20px;
    border-radius: 15px;
    box-shadow: 0 10px 25px rgba(0,0,0,0.1);
    width: 350px;
    text-align: center;
}

h1 {
    margin-bottom: 20px;
    font-size: 1.5rem;
}

/* Mode Selector */
.mode-selector {
    display: flex;
    justify-content: center;
    margin-bottom: 20px;
    gap: 10px;
}

.mode-btn {
    padding: 8px 20px;
    border: none;
    background-color: var(--btn-bg);
    color: var(--text-color);
    border-radius: 20px;
    cursor: pointer;
}

.mode-btn.active {
    background-color: var(--accent-color);
    color: var(--accent-text);
}

/* Calculator Area */
.calculator {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.hidden {
    display: none;
}

.display {
    width: 100%;
    height: 60px;
    background-color: var(--display-bg);
    color: var(--text-color);
    border: none;
    border-radius: 10px;
    font-size: 1.5rem;
    text-align: right;
    padding: 10px;
    margin-bottom: 10px;
}

/* Buttons Grid */
.buttons-grid {
    display: grid;
    gap: 10px;
}

.simple-grid {
    grid-template-columns: repeat(4, 1fr);
}

.advanced-grid {
    grid-template-columns: repeat(4, 1fr);
}

button {
    padding: 15px;
    font-size: 1.1rem;
    border: none;
    border-radius: 8px;
    background-color: var(--btn-bg);
    color: var(--text-color);
    cursor: pointer;
}

button:hover {
    background-color: var(--btn-hover);
}

.span-2 {
    grid-column: span 2;
}

.equal-btn {
    background-color: var(--operator-color);
    color: #fff;
    grid-row: span 2; 
    /* Ensures equal button is tall in simple mode */
    height: 100%;
}

/* Specific styling for advanced equal button to not be tall if needed */
.advanced-grid .equal-btn {
    grid-row: span 1;
}