chore: Add color prop to container style API

[jkuelz]

Description

Adds color prop to container's style.root property.

We allow changing the background color, so to enable accessible color combinations we should allow the default color to be changed as well. This allows components like Box to be used with color="inherit" and inherit the color defined.

P0 item from Venue sign-off.

Related links, issue #, if available: QuipZjcAA3fghsSF

How has this been tested?

Added an example to the custom-styles dev page for visual regression testing.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[mxschll]

The snapshots need updating 😞