Fix colour contrast issues throughout `src/theme/` to meet WCAG AA

[success-OG]

Labels: frontend accessibility

Background
docs/ACCESSIBILITY.md documents a WCAG AA compliance target. Several colour combinations in src/theme/ fail the 4.5:1 contrast ratio for normal text. Secondary text and placeholder colours are the main offenders.

What "Done" Looks Like

• Audit all colour pairs in src/theme/ using a contrast ratio calculator
• Fix all pairs that fail 4.5:1 (normal text) or 3:1 (large text / UI components)
• Do not change primary brand colours — adjust secondary/tertiary shades only
• Update src/theme/index.ts and verify changes in both light and dark mode
• Include a contrast ratio table in the PR description showing before/after values

[ajaheberechukwulilian-2265]

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[Silver36-ship-it]

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[levoski1]

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[Ceejaytech25]

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GM GM Maintainer, pls can I work on this issue ? ETA - 24hrs

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This issue has been completed by @Ceejaytech25 as part of the Stellar Wave Program's 6th Wave 🥳

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