fix font size change bug

packages/cells/src/widget.ts

@@ -364,6 +364,14 @@ export class Cell extends Widget {
     this.editor.focus();
   }
 
+  /**
+   * Handle `fit-request` messages.
+   */
+  protected onFitRequest(msg: Message): void {
+    // need this for for when a theme changes font size
+    this.editor.refresh();
+  }
+
   /**
    * Handle `update-request` messages.
    */