IntelliJ Themes: fixed window title bar background if unified background is enabled

flatlaf-core/src/main/resources/com/formdev/flatlaf/IntelliJTheme$ThemeLaf.properties

@@ -22,6 +22,12 @@
 #  - https://www.formdev.com/flatlaf/how-to-customize/
 #
 
+#---- system colors ----
+
+# fix system colors because they are usually not set in .json files
+control = lazy(Panel.background)
+
+
 #---- Button ----
 
 Button.startBackground = $Button.background