allow theming of single tray Icon

Is your feature request related to a problem? I have an application that has a tray icon that has nearly the same color as my tray background. The Application also changes tray icons based on the state.

Describe the solution you'd like I want to theme this specific tray icon with another background color. Currently I can not select the tray Icon of a specific application. If the widget of this icon would have a ID or name based on the application it belogs to (like the "icon" option) I could select it with CSS and style it seperately.

Describe alternatives you've considered Alternatively there could be an option like "icon" but to define a css class that the widget of the given application should be marked with.

I can not use the "icon" option for the tray module because the Icons change over time.

Additional context