[Manual Backport 2.x] feat: Add V9 Theme (#7757)

[virajsanghvi]

Description

Manual backport as user specific themes wasn't on 2.x causing some changes.

Main difference of this is that changes in ui_render_mixin (https://github.com/opensearch-project/OpenSearch-Dashboards/pull/7757/files#diff-b524e9b1aa714d4d29d9295214b20ea793c69d8e254203c623d32fe13a14d972) are now in src/core/server/rendering/views/fonts.tsx and src/core/server/rendering/views/theme.ts. Also included a docs update to document difference.

Issues Resolved

N/A

Screenshot

N/A

Testing the changes

Tested this locally and changing theme in advanced settings has expected behavior (and correct files load).

Changelog

• skip

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[github-actions]

❌ Invalid Additional Prefix With Skip Entry Option

If your Changelog section includes the 'skip' entry option, it cannot also include other changelog entry prefixes. The option 'skip' must be alone when used. Please review your Changelog section again.

[github-actions]

❌ Invalid Additional Prefix With Skip Entry Option

If your Changelog section includes the 'skip' entry option, it cannot also include other changelog entry prefixes. The option 'skip' must be alone when used. Please review your Changelog section again.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.87%. Comparing base (1987d5c) to head (78649ff).
Report is 2 commits behind head on 2.x.

Files	Patch %	Lines
src/core/server/ui_settings/settings/theme.ts	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #7775   +/-   ##
=======================================
  Coverage   63.86%   63.87%           
=======================================
  Files        3653     3655    +2     
  Lines       80992    81045   +53     
  Branches    12885    12894    +9     
=======================================
+ Hits        51728    51769   +41     
- Misses      26099    26107    +8     
- Partials     3165     3169    +4     

Flag	Coverage Δ
Linux_1	30.10% <23.07%> (-0.02%)	⬇️
Linux_2	55.95% <92.30%> (-0.03%)	⬇️
Linux_3	40.52% <23.07%> (-0.02%)	⬇️
Linux_4	31.46% <23.07%> (-0.03%)	⬇️
Windows_1	30.12% <23.07%> (?)
Windows_2	55.91% <92.30%> (-0.03%)	⬇️
Windows_3	40.53% <23.07%> (?)
Windows_4	31.46% <23.07%> (-0.03%)	⬇️

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