Minor single doc mode styling

packages/application/style/base.css

@@ -35,6 +35,10 @@ body {
   padding: 0;
 }
 
+#jp-main-dock-panel[data-mode='single-document'] .jp-MainAreaWidget {
+  border: none;
+}
+
 #jp-top-panel {
   border-bottom: var(--jp-border-width) solid var(--jp-border-color0);
   background: var(--jp-layout-color1);

packages/apputils/src/toolbar.tsx

@@ -70,8 +70,10 @@ class ToolbarLayout extends PanelLayout {
       // accommodate them.
       if (some(this.widgets, w => !w.isHidden)) {
         this.parent!.node.style.minHeight = 'var(--jp-private-toolbar-height)';
+        this.parent!.removeClass('jp-Toolbar-micro');
       } else {
         this.parent!.node.style.minHeight = '';
+        this.parent!.addClass('jp-Toolbar-micro');
       }
     }
 

packages/apputils/style/toolbar.css

@@ -103,3 +103,12 @@ button.jp-ToolbarButtonComponent .jp-ToolbarButtonComponent-label {
   padding-left: 2px;
   color: var(--jp-ui-font-color1);
 }
+
+#jp-main-dock-panel[data-mode='single-document']
+  .jp-MainAreaWidget
+  > .jp-Toolbar.jp-Toolbar-micro {
+  padding: 0;
+  border: none;
+  box-shadow: none;
+  min-height: 0;
+}