[NeutralColor]: Add possibility to change all Neutral Color/Greys with createLightTheme/createDarkTheme

Library

React Components / v9 (@fluentui/react-components)

Describe the feature that you would like added

Add possibility to change all Neutral Color/Greys with createLightTheme/createDarkTheme.

Actually, method createLightTheme/createDarkTheme do not provided any possibility to change Neutral colors (like Fluent UI8).

It's a problem if the brand color contains a colored grey + primary color.

The only current way i finded, is create a complete Fluent UI 8 theme (https://codepen.io/pen) and use createV9Theme from @fluentui/react-migration-v8-v9 package

It's not a good way.

Possible solution :

Pass a greys Ramp to method.

createLightTheme: (brand: BrandVariants, greys? : Greys) => Theme

PS : For me it'a bug, i have marked this as feature

Have you discussed this feature with our team

No response

Additional context

No response

Validations

• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Priority

Normal

Start with the createLightTheme and createDarkTheme entry points in @fluentui/react-components, then compare their neutral-color handling with the Fluent UI 8 theme and the createV9Theme migration path. Done means callers can provide an optional Greys ramp and have the light or dark theme use those neutral colors, with the behavior covered by the relevant theme tests.