:root {
    --primary-bg-color: #1a1a2e;
    --secondary-bg-color: #16213e;
    --tertiary-bg-color: #0f3460;
    --primary-text-color: #e94560;
    --secondary-text-color: #dcdcdc;
    --accent-color: #e94560;
    --button-bg-color: #e94560;
    --button-text-color: #ffffff;
    --shadow-color: rgba(233, 69, 96, 0.4);
}

body {
    font-family: 'Poppins', sans-serif;
    background-color: var(--primary-bg-color);
    color: var(--secondary-text-color);
    margin: 0;
    padding: 20px;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
}

#app-container {
    width: 100%;
    max-width: 1200px;
    background-color: var(--secondary-bg-color);
    border-radius: 20px;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.3);
    overflow: hidden;
}

header {
    background: linear-gradient(45deg, var(--tertiary-bg-color), var(--accent-color));
    color: white;
    text-align: center;
    padding: 20px;
    text-shadow: 0 2px 4px rgba(0,0,0,0.2);
}

header h1 {
    margin: 0;
    font-weight: 600;
}

header p {
    margin: 5px 0 0;
    font-weight: 300;
}

main {
    display: flex;
    padding: 20px;
    gap: 20px;
}

#controls-panel {
    width: 300px;
    display: flex;
    flex-direction: column;
    gap: 20px;
}

#content-display {
    flex: 1;
    display: flex;
    gap: 20px;
    min-width: 0;
}

.button {
    background-color: var(--button-bg-color);
    color: var(--button-text-color);
    border: none;
    padding: 10px 15px;
    border-radius: 8px;
    cursor: pointer;
    font-weight: 600;
    transition: all 0.3s ease;
    box-shadow: 0 4px 15px var(--shadow-color);
}

.button:hover, .button:focus {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px var(--shadow-color);
    filter: brightness(1.1);
}

#file-upload-area {
    background: var(--tertiary-bg-color);
    padding: 20px;
    border-radius: 12px;
    text-align: center;
}

#file-name {
    margin-top: 10px;
    font-size: 0.9em;
    word-break: break-all;
}

#tts-controls, #api-key-area {
    background: var(--tertiary-bg-color);
    padding: 20px;
    border-radius: 12px;
}

h2 {
    color: var(--primary-text-color);
    margin-top: 0;
    border-bottom: 2px solid var(--accent-color);
    padding-bottom: 10px;
    margin-bottom: 20px;
}

.control-group {
    margin-bottom: 15px;
}

.control-group label {
    display: block;
    margin-bottom: 5px;
    font-weight: 600;
}

select, input[type="range"], input[type="password"] {
    width: 100%;
    padding: 8px;
    border-radius: 5px;
    border: 1px solid var(--tertiary-bg-color);
    background-color: var(--primary-bg-color);
    color: var(--secondary-text-color);
}

#speed-control {
    -webkit-appearance: none;
    appearance: none;
    height: 8px;
    background: var(--primary-bg-color);
    outline: none;
    border-radius: 5px;
}

#speed-control::-webkit-slider-thumb {
    -webkit-appearance: none;
    appearance: none;
    width: 20px;
    height: 20px;
    background: var(--accent-color);
    cursor: pointer;
    border-radius: 50%;
}

#api-key-area .control-group {
    display: flex;
    gap: 10px;
}

#chapter-list {
    background: var(--tertiary-bg-color);
    padding: 20px;
    border-radius: 12px;
    width: 200px;
}

#chapters {
    list-style: none;
    padding: 0;
    margin: 0;
    max-height: 400px;
    overflow-y: auto;
}

#chapters li {
    padding: 10px;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.2s;
}

#chapters li:hover {
    background-color: var(--secondary-bg-color);
}

#chapters li.active {
    background-color: var(--accent-color);
    color: white;
    font-weight: 600;
}

#text-display-area {
    flex: 1;
    background: var(--tertiary-bg-color);
    padding: 20px;
    border-radius: 12px;
    min-width: 0;
    overflow-y: auto;
    max-height: 500px;
}

#text-content {
    font-size: 1.1em;
    line-height: 1.6;
}

footer {
    background-color: var(--tertiary-bg-color);
    padding: 20px;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

#playback-controls {
    display: flex;
    gap: 15px;
}

#playback-controls .button {
    font-size: 1.2em;
    padding: 12px 20px;
}

/* --- Responsive Design for Mobile --- */
@media (max-width: 768px) {
    body {
        padding: 0;
    }

    #app-container {
        border-radius: 0;
        min-height: 100vh;
    }

    main {
        flex-direction: column;
        padding: 10px;
    }

    #controls-panel {
        width: 100%;
    }

    #content-display {
        flex-direction: column;
    }

    #chapter-list {
        width: 100%;
        max-height: 150px; /* Make chapter list smaller on mobile */
    }

    header h1 {
        font-size: 1.5em;
    }

    #text-content {
        font-size: 1em;
    }

    footer {
        flex-direction: column;
        gap: 15px;
    }
}