:root {
    --font-sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    --header-bg: #f6f8fa;
    --bg-color: #ffffff;
    --border-color: #d0d7de;
    --text-color: #24292e;
    --icon-color: #57606a;
    --active-color: #0969da;
    --tab-hover-bg: #eef1f4;
}
body.dark-mode {
    --header-bg: #161b22;
    --bg-color: #0d1117;
    --border-color: #30363d;
    --text-color: #c9d1d9;
    --icon-color: #8b949e;
    --active-color: #58a6ff;
    --tab-hover-bg: #21262d;
}

/* --- Base Layout --- */
body {
    margin: 0;
    font-family: var(--font-sans);
    display: flex;
    flex-direction: column;
    height: 100vh;
    overflow: hidden;
    background-color: var(--bg-color);
}
.main-header {
    display: flex;
    flex-shrink: 0;
    background-color: var(--header-bg);
    border-bottom: 1px solid var(--border-color);
}
.main-content {
    flex-grow: 1;
    display: flex;
    min-height: 0;
}
.editor-pane, .preview-pane {
    flex: 1;
    height: 100%;
    overflow: auto;
    min-width: 0;
}
.resizer {
    flex: 0 0 5px;
    cursor: col-resize;
    background-color: var(--border-color);
}

/* --- Tabs --- */
.tabs-container {
    flex-grow: 1;
    display: flex;
    align-items: flex-end;
    overflow-x: auto;
}
.tab {
    display: flex; align-items: center; gap: 8px;
    padding: 10px 12px;
    cursor: pointer;
    border-right: 1px solid var(--border-color);
    color: var(--icon-color);
    white-space: nowrap;
}
.tab:hover { background-color: var(--tab-hover-bg); }
.tab.active {
    background-color: var(--bg-color);
    color: var(--text-color);
    border-top: 2px solid var(--active-color);
    margin-bottom: -1px;
}
.tab-close-btn {
    background: none; border: none; font-size: 20px; line-height: 1;
    cursor: pointer; padding: 2px; border-radius: 4px;
    color: var(--icon-color); opacity: 0.6;
}
.tab:hover .tab-close-btn { opacity: 1; }
.add-tab-btn {
    display: flex; align-items: center; justify-content: center;
    width: 40px; align-self: center; font-size: 22px;
    cursor: pointer; border: none; background: none;
    color: var(--icon-color);
    border-left: 1px solid var(--border-color);
    margin-left: 4px;
}

/* --- Controls --- */
.controls-container-desktop {
    display: flex;
    align-items: center;
    padding: 0 10px;
    border-left: 1px solid var(--border-color);
}
.controls-container-mobile { display: none; }
.control-btn {
    display: flex; align-items: center; gap: 6px;
    padding: 8px; margin-left: 5px;
    border: none; border-radius: 6px;
    background-color: transparent;
    cursor: pointer;
    color: var(--text-color);
}
.control-btn:hover { background-color: var(--tab-hover-bg); }
.control-btn.active { background-color: var(--active-color); color: white; }
.control-btn.active svg { fill: white; }
.control-btn svg { width: 18px; height: 18px; fill: var(--icon-color); }

/* --- Mobile Specific Layout --- */
@media (max-width: 768px) {
    .main-content { flex-direction: column; margin-bottom: 50px; }
    .resizer { width: 100%; height: 8px; cursor: row-resize; }
    .controls-container-desktop { display: none; }
    .controls-container-mobile {
        position: fixed;
        bottom: 0; left: 0; right: 0;
        display: flex;
        justify-content: space-around;
        height: 50px;
        background-color: var(--header-bg);
        border-top: 1px solid var(--border-color);
        z-index: 100;
    }
    .control-btn span { display: none; }
}

/* --- CodeMirror & Iframe --- */
.CodeMirror { height: 100%; font-size: 14px; }
iframe { width: 100%; height: 100%; border: none; background-color: #fff; }

/* --- Fullscreen Mode --- */
body.fullscreen-active .main-header,
body.fullscreen-active .editor-pane,
body.fullscreen-active .resizer,
body.fullscreen-active .controls-container-mobile {
    display: none;
}
body.fullscreen-active .main-content { display: block; margin-bottom: 0; }
body.fullscreen-active .preview-pane { width: 100vw; height: 100vh; }