Banner: create providers for display

[danirabbit]

Fixes deprecated adding provider to style context

If a brand color is available, creates a style provider with a css class for that color and adds it to a list of providers. Doesn't add a provider if no brand color available. Re-uses providers if two apps use the same brand color

[jeremypw]

This differs from main for applications that have not set a custom background (?) e.g. Image Optimizer. For these, the banner has no background color whereas in main it uses the user's accent color.

[danirabbit]

@jeremypw is this different from what you're seeing?