Fix Settings tab color mismatch with Settings page background (Dark/Light only)

[Charan.k (k-charansai)]

Summary

This fixes the Settings tab color mismatch, addressing the visual inconsistency where the Settings tab appeared black while the Settings page was dark gray.

Looking at the history of the original fix (#19999) and its subsequent revert (#20292 / #20186), it appears the accessibility/contrast concern was isolated to the HighContrast theme change (SystemControlBackgroundBaseLowBrush → SystemColorWindowBrush).

To address the original issue while avoiding the accessibility regression, this patch reapplies only the Dark and Light theme softening and leaves the HighContrast theme exactly as it is on main today.

Changes

• Dark Theme: Aligned SettingsUiTabBrush from #0c0c0c to #282828 to match the Settings page background.
• Light Theme: Aligned SettingsUiTabBrush from #ffffff to #F9F9F9.
• HighContrast Theme: Left entirely unchanged.

Closes #19873

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