Fix scoped labels color regression in dark mode

What does this MR do?

• Updates "_dark.scss" to correctly display label texts both in normal and dark mode
  • Use white-ish($gray-900: #fafafa) text in scoped labels.

This update fixes #270389 (closed).

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Screenshots (strongly suggested)

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-	Screen_Recording_2020-11-17_at_1.49.22_PM
_	Screen_Recording_2020-11-17_at_1.47.51_PM

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team