Update the Modal Heading and Label text color value

[falgunihdesai]

Update the Modal Heading and Label text color value so that it matches with the text color used for other areas of the editor. Previously it was using color from wp-admin common.css. Fixes #50448

What?

Why?

How?

Testing Instructions

1. Open the WordPress page editor.2.
2. Open the Preferences modal and check the colour value using inspect element.

Testing Instructions for Keyboard

Screenshots or screencast

[jorgefilipecosta]

I'm not seeing regressions and this seems to work. Thank you for this PR @falgunihdesai.

[jorgefilipecosta]

I'm going to cc: @richtabor to double-check if this is the expected result from the UX point of view.

[github-actions]

Warning: Type of PR label error

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[richtabor]

LGTM