Inspect the webpage in your browser and see the hierarchy of classes in which the background color is changed.
Copy the classes. Give a new class name(example: custom-nav-color) for the Navigation tree widget in your studio pro.
The best way is to override the default behavior in your custom CSS file ( You can add it if you do not have one).
In your custom SCSS file write your custom class name which is custom-nav-color and under this class paste the Mendix default hierarchy classes which we copied from the browser.
By this way we are introducing a parent class and targeting the Mendix default classes and overriding the default style.
Note: DO NOT OVERRIDE THE CLASSES FROM THE MENDIX-GENERATED DEFAULT SCSS FILES :)