It isn’t possible to change the background of the toolbar currently, as it is controlled by the operating system and what the user has selected on each device under the View menu in Customize Toolbar. The toolbar is impacted only by using light or dark mode. We have had other people request the ability to customize the color of the toolbar, but it isn’t currently possible beyond the fact we comply with light/dark themes. So either the toolbar is in dark or light mode based on your style. That’s the limit of what you can control with styles.
If you’d like to request some specific areas be added in the future, the best way is to send us an email at email@example.com where our support humans kindly make sure all requests are properly recorded.