Improve RSVP focus-letter colour contrast

[Steadyx]

What changed

Make the RSVP focus letter stand out more clearly from the page and surrounding text using a richer theme-derived colour and a higher contrast target. Extend colour regression checks and device coverage across all built-in themes.

Risk

Limited to RSVP accent colours and their tests. Custom palettes retain legibility checks; reading timing, positions, and navigation are unchanged.

Validation

Local qualityGate passed, including lint and the 914-test unit suite. All five focused device tests passed on CPH2493 / Android 16. Commit and push hooks passed.

• ./gradlew qualityCheck
• ./gradlew qualityGate when Android/UI/build behavior changed
• Android Lint attempted for Android-facing changes; any analyzer stall is documented separately
• Focused regression tests added or updated
• git diff --check
• Connected instrumentation run when device behavior changed, or the missing device/emulator is documented

Quality policy

• No baseline, ignoreFailures, broad exclusion, or unrelated threshold relaxation was added
• Any new suppression is declaration-scoped and explained next to the annotation
• No generated files, debug logging, commented-out code, or unrelated formatting churn is included