Allow associating CSS class(es) with a router outlet

[danielwiehl]

Is your feature request related to a problem? Please describe.

The SCION Workbench integrates microfrontends in different places, such as views and overlays. The microfrontends have a different background color depending on where they are embedded. For example, in dark mode, the background color of overlays is slightly brighter, or views in the peripheral area have a different background color than views in the main area.

Describe the solution you'd like

The router outlet should provide API to associate CSS class(es) with the embedded microfrontend, enabling a microfrontend to apply styles based on its embedding context. The CSS class(es) should be set on the HTML root element of the microfrontend.

Describe alternatives you've considered

The application can set and evaluate CSS classes via context values. However, the CSS classes must be received and set by each microfrontend. Therefore, setting CSS classes should be built into the platform.