[UI Shell] Header action color update

[thyhmdo]

Currently, the Selected state doesn't reflect the color correctly.

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The correct color token: $layer

[Storybook link ](https://react.carbondesignsystem.com/?path=/story/components-ui-shell-header--header-w-actions-and-right-panel)

Need to update

• https://carbondesignsystem.com/components/UI-shell-header/style/#header-actions