docs: correct dark-mode brand-primary and brand-border values in color reference

[detail-app]

Updates four stale Dark-token rows in openmetadata-ui/.../docs/colors.md
(Color Usage Handbook) so they match the dark palette shifted by 5ffed97:

• §2.1 tw:bg-brand-primary dark: "Brand 500 at 14%" -> "Brand 600 at 20%"
• §2.3 tw:border-brand dark: #53b1fd (brand-400) -> #1570ef (brand-600)
• §2.6 tw:bg-brand-primary dark: #2e90fa (brand-500) -> #1570ef (brand-600)
• §2.6 tw:border-brand dark: #53b1fd (brand-400) -> #1570ef (brand-600)
  (the source tokens in globals.css already changed; the spec files were
  already updated by the same commit; only this living color handbook lagged).

Example: a developer styling a focused input with tw:border-brand in dark
mode following the handbook would expect a brand-400-blue border (#53b1fd),
but since 5ffed97 actually renders #1570ef (brand-600). Likewise a new brand
tint surface built off the §2.1 row would have been designed against the old
Brand 500 at 14% instead of the current Brand 600 at 20%, looking washed
out against the rest of the dark UI palette.

Introduced by commit 5ffed97 (@shah-harshit, #35290)

Describe your changes:

Fixes # (no issue — pure doc drift raised by the change).

I corrected four stale dark-mode token values in the Color Usage Handbook
(openmetadata-ui/src/main/resources/ui/docs/colors.md) because commit
5ffed97 shifted --color-bg-brand-primary and --color-border-brand dark
values in globals.css but the handbook still listed the old recipes/hexes.

Type of change:

• Documentation

High-level design:

N/A — single-file doc sync (4 rows across 3 sections of one handbook).

Tests:

Not applicable — no code change; values verified against globals.css dark
overrides and the contract test (theme-contract.test.js) updated by the
same commit.

UI screen recording:

Not applicable — documentation-only change, no UI behavior or styling change.

Checklist:

• I have read the CONTRIBUTING document.
• My PR is linked to a GitHub issue via Fixes #<issue-number> above. (N/A — doc drift, no issue.)
• I have added tests (unit / integration / Playwright as applicable) and listed them above. (N/A — docs-only.)

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[github-actions]

Hi there 👋 Thanks for your contribution!

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[shah-harshit]

Folded into #35294 (commit 8e149d4), which also owns the dark token sources. The bg-brand-primary quick-reference row there reads "Brand 600 at 20%" rather than a solid #1570ef, matching globals.css. Closing in favour of #35294.