:root {
    --bg-color: #131722;
    --pane-bg-color: #1e222d;
    --text-color: #d1d4dc;
    --border-color: #363a45;
    --hover-color: #2a2e39;
    --active-color: #2962ff;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    margin: 0;
    height: 100vh;
    overflow: hidden;
}

.app-container {
    display: flex;
    flex-direction: column;
    height: 100vh;
}

.top-toolbar {
    display: flex;
    align-items: center;
    padding: 0 10px;
    height: 40px;
    background-color: var(--pane-bg-color);
    border-bottom: 1px solid var(--border-color);
}

.toolbar-item {
    padding: 8px 12px;
    cursor: pointer;
    font-size: 14px;
}

.toolbar-item:hover {
    background-color: var(--hover-color);
    border-radius: 4px;
}

.main-layout {
    display: flex;
    flex-grow: 1;
    overflow: hidden;
}

.left-sidebar, .right-sidebar {
    width: 280px; /* Slightly wider sidebar */
    background-color: var(--pane-bg-color);
    padding: 15px;
    display: flex;
    flex-direction: column;
    overflow-y: auto;
    flex-shrink: 0; /* Prevent sidebars from shrinking */
}

.left-sidebar {
    border-right: 1px solid var(--border-color);
}

.right-sidebar {
    border-left: 1px solid var(--border-color);
}

.left-sidebar h2, .right-sidebar h2 {
    margin-top: 0;
    font-size: 18px;
    border-bottom: 1px solid var(--border-color);
    padding-bottom: 10px;
    margin-bottom: 10px;
}

#curriculum-list {
    list-style: none;
    padding: 0;
    margin: 0;
}

/* Step Header Styling */
.step-header {
    padding: 10px;
    cursor: pointer;
    font-size: 16px;
    font-weight: bold;
    color: var(--text-color);
    border-radius: 4px;
    margin-top: 5px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    background-color: var(--border-color); /* Slightly darker background for headers */
    user-select: none; /* Prevent text selection on click */
}

.step-header:hover {
    background-color: var(--hover-color);
}

.step-header::after {
    content: '▶'; /* Right arrow for collapsed */
    font-size: 12px;
    margin-left: 10px;
    transition: transform 0.2s ease-in-out;
}

.step-header.expanded::after {
    content: '▼'; /* Down arrow for expanded */
    transform: rotate(0deg); /* Explicitly set for consistency */
}

/* Sub-Lesson List Styling */
.sub-lesson-list {
    list-style: none;
    padding: 0;
    margin: 0;
    margin-left: 15px; /* Indent sub-lessons */
    max-height: 500px; /* Max height for transition - adjust as needed */
    overflow: hidden; /* Hide overflow when collapsed */
    transition: max-height 0.3s ease-out, opacity 0.3s ease-out; /* Smooth transition */
    opacity: 1;
}

.sub-lesson-list.collapsed {
    max-height: 0;
    opacity: 0;
    visibility: hidden; /* Ensure it's not interactive when collapsed */
}

/* Lesson Item Styling */
.lesson-item {
    padding: 8px 10px;
    cursor: pointer;
    font-size: 14px;
    border-radius: 4px;
    margin-top: 2px;
    color: var(--text-color);
}

.lesson-item:hover {
    background-color: var(--hover-color);
}

.lesson-item.active {
    background-color: var(--active-color);
    color: #fff;
    font-weight: bold;
}

/* Main Content and Chart */
.main-content {
    flex-grow: 1;
    position: relative; /* For chart positioning */
    background-color: var(--bg-color); /* Chart background */
}

#chart-container {
    position: absolute;
    width: 100%;
    height: 100%;
}

/* Lesson Details Panel */
#lesson-details h3 {
    font-size: 16px;
    margin-top: 0;
    margin-bottom: 10px;
    color: var(--text-color);
}

#lesson-details p {
    font-size: 14px;
    line-height: 1.5;
    color: #b2b5be;
}