A frequent request with the COMBOBOX is to alter its appearance to make it more obvious that it is active. Particularly in Windows 10 where its background color is the same as an active BUTTON, which is not the same as an active EDIT widget.
You could follow the advice in an earlier Ask Reuben
https://4js.com/ask-reuben/ig-10/ and use Pseudo Selectors to override the native appearance when a widget is active and/or inactive IF the native widget allows it. For ComboBox you can use another technique where we render an alternative ComboBox that we have made available via presentation styles.
Read more at
https://4js.com/ask-reuben/ig-27/