feat(toolbar): Better theming

[tomasdev]

Feature Description

It would be great if theming was simpler overall. But specifically for the mat-toolbar component, the SASS looks convoluted to be able to just change the background color to something other than the accent/primary/warn settings.

Proposed Solution

Instead of having to do

@include mat-toolbar-theme(
  map_merge(
    $theme, (
      background: map_merge(
        map-get($theme, background), (
          app-bar: #fff,
        )
      )
    )
  )
);

Have something like:

@include mat-toolbar-theme($theme, (app-bar: #fff));

Implementation can vary widely, but overall I'd like to have mat-toolbar-theme($theme, $extra... = ()) for extensibility.

Looking forward to reading your thoughts!

[tomasdev]

Duplicate of #6244

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