Merge pull request #3344 from meeseeksmachine/auto-backport-of-pr-334…

…2-on-7.x

Backport PR #3342 on branch 7.x (HTMLManager: Do not reset lab's CSS variables if they are defined)

packages/html-manager/src/libembed.ts

@@ -6,7 +6,16 @@ __webpack_public_path__ = (window as any).__jupyter_widgets_assets_path__ || __w
 
 import 'font-awesome/css/font-awesome.css';
 import '@lumino/widgets/style/index.css';
-import '@jupyter-widgets/controls/css/widgets.css';
+import '@jupyter-widgets/controls/css/widgets-base.css';
+
+// If lab variables are not found, we set them (we don't want to reset the variables if they are already defined)
+if (
+  getComputedStyle(document.documentElement).getPropertyValue(
+    '--jp-layout-color0'
+  ) === ''
+) {
+  require('@jupyter-widgets/controls/css/labvariables.css');
+}
 
 // Used just for the typing. We must not import the javascript because we don't
 // want to include it in the require embedding.