:root {
    --bg-color: #f8f9fa;
    --container-bg: white;
    --text-color: #333;
    --heading-color: #2c3e50;
    --section-bg: #f1f8ff;
    --border-color: #e1e4e8;
    --button-primary: #3498db;
    --button-primary-hover: #2980b9;
    --button-secondary: #95a5a6;
    --button-secondary-hover: #7f8c8d;
    --delete-button: #e74c3c;
}

.dark-mode {
    --bg-color: #121212;
    --container-bg: #1e1e1e;
    --text-color: #e0e0e0;
    --heading-color: #58b5ff;
    --section-bg: #252525;
    --border-color: #444;
    --button-primary: #2980b9;
    --button-primary-hover: #3498db;
    --button-secondary: #7f8c8d;
    --button-secondary-hover: #95a5a6;
    --delete-button: #c0392b;
}

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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    line-height: 1.6;
    color: var(--text-color);
    background-color: var(--bg-color);
    padding: 20px;
    transition: background-color 0.3s ease, color 0.3s ease;
}

.container {
    max-width: 800px;
    margin: 0 auto;
    background: var(--container-bg);
    padding: 30px;
    border-radius: 10px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
    transition: background-color 0.3s ease;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 30px;
}

.theme-toggle {
    background: none;
    border: none;
    cursor: pointer;
    font-size: 1.5rem;
    padding: 5px 10px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background-color 0.3s;
    background-color: var(--section-bg);
}

.theme-toggle:hover {
    background-color: var(--border-color);
}

h1 {
    text-align: center;
    color: var(--heading-color);
}

h2, h3 {
    color: var(--heading-color);
    margin: 20px 0 15px;
}

.main-section {
    display: flex;
    flex-direction: column;
    align-items: center;
    margin-bottom: 40px;
    padding: 20px;
    background-color: var(--section-bg);
    border-radius: 8px;
    transition: background-color 0.3s ease;
}

.activity-display {
    min-height: 100px;
    width: 100%;
    padding: 20px;
    margin-bottom: 20px;
    background-color: var(--container-bg);
    border-radius: 8px;
    border: 1px solid var(--border-color);
    display: flex;
    align-items: center;
    justify-content: center;
    text-align: center;
    font-size: 1.2em;
    transition: background-color 0.3s ease, border-color 0.3s ease;
}

.youtube-container {
    margin: 20px 0;
    width: 100%;
    max-width: 560px;
    aspect-ratio: 16/9;
}

.youtube-container iframe {
    width: 100%;
    height: 100%;
    border-radius: 8px;
}

.button {
    padding: 10px 20px;
    background-color: var(--button-primary);
    color: white;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    font-size: 16px;
    transition: background-color 0.3s;
}

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

.button.secondary {
    background-color: var(--button-secondary);
}

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

.input-group {
    display: flex;
    margin-bottom: 20px;
    gap: 10px;
}

.input-group input {
    flex-grow: 1;
    padding: 10px;
    border: 1px solid var(--border-color);
    border-radius: 5px;
    font-size: 16px;
    background-color: var(--container-bg);
    color: var(--text-color);
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

.activity-list {
    list-style-type: none;
    margin-bottom: 20px;
    max-height: 200px;
    overflow-y: auto;
    border: 1px solid var(--border-color);
    border-radius: 5px;
    padding: 10px;
    transition: border-color 0.3s ease;
}

.activity-list li {
    padding: 8px 10px;
    display: flex;
    justify-content: space-between;
    border-bottom: 1px solid var(--border-color);
    transition: border-color 0.3s ease;
}

.activity-list li:last-child {
    border-bottom: none;
}

.activity-list button {
    background: none;
    border: none;
    color: var(--delete-button);
    cursor: pointer;
    font-size: 18px;
}

.actions {
    display: flex;
    justify-content: space-between;
    margin-top: 20px;
}

@media (max-width: 600px) {
    .youtube-container {
        width: 100%;
    }

    .input-group {
        flex-direction: column;
    }
}