Index: /trunk/src/org/openstreetmap/josm/gui/MainApplication.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/MainApplication.java	(revision 17795)
+++ /trunk/src/org/openstreetmap/josm/gui/MainApplication.java	(revision 17796)
@@ -905,4 +905,6 @@
         // Configure Look and feel before showing SplashScreen (#19290)
         setupUIManager();
+        // MainFrame created before setting look and feel and not updated (#20771)
+        SwingUtilities.updateComponentTreeUI(mainFrame);
 
         final SplashScreen splash = GuiHelper.runInEDTAndWaitAndReturn(SplashScreen::new);