Adjust tokens and validate contrast per WCAG AA.

Theme Variables
--background
--foreground
--card
--card-foreground
--primary
--primary-foreground
--secondary
--secondary-foreground
--muted
--muted-foreground
--accent
--accent-foreground
--destructive
--destructive-foreground
--border
--input
--ring
Custom Color Schemes
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Import (JSON)
Merges with existing schemes.