💄 Finished the common style settings.

assets/css/_common/outline/sidebar/sidebar-nav.scss

@@ -0,0 +1,57 @@
+// Sidebar Navigation
+.sidebar-nav {
+  display: none;
+  margin: 0;
+  padding-bottom: 20px;
+  padding-left: 0;
+
+  .sidebar-nav-active & {
+    display: block;
+  }
+
+  li {
+    border-bottom: 1px solid transparent;
+    color: $sidebar-nav-color;
+    cursor: pointer;
+    display: inline-block;
+    font-size: $font-size-small;
+
+    &.sidebar-nav-overview {
+      margin-left: 10px;
+    }
+
+    &:hover {
+      color: $sidebar-nav-hover-color;
+    }
+  }
+}
+
+.sidebar-toc-active .sidebar-nav-toc, .sidebar-overview-active .sidebar-nav-overview {
+  border-bottom-color: $sidebar-highlight;
+  color: $sidebar-highlight;
+
+  &:hover {
+    color: $sidebar-highlight;
+  }
+}
+
+// Need for Sidebar/TOC inner scrolling if content taller then viewport.
+.sidebar-panel-container {
+  flex: 1;
+  overflow-x: hidden;
+  overflow-y: auto;
+}
+
+.sidebar-panel {
+  display: none;
+}
+
+.sidebar-overview-active .site-overview-wrap {
+  // Flexbox layout makes it possible to reorder the child
+  // elements of .site-overview-item through the `order` CSS property
+  @include flex-column();
+}
+
+.sidebar-toc-active .post-toc-wrap {
+  display: block;
+}