[Manual Backport 2.x] refactor: simplify theme configuration and defaulting (#7625)

[virajsanghvi]

Backport from #7625

---

Signed-off-by: Viraj Sanghvi [email protected]
Co-authored-by: opensearch-changeset-bot[bot] <154024398+opensearch-changeset-bot[bot]@users.noreply.github.com>
(cherry picked from commit bcdbbef)

Description

The backport failed because user specific themes was pulled from 2.x. Some feedback says it was due to the UX, #6978 says due to testing, and others because it caused a blank white screen. Because I'm not sure that'll get resolved, I changed my PR to handle the differences.

The main callouts are that:

• The changes to ui_render_mixin and the bootstrap and startup template files are now just within the ui_render_mixin and bootstrap file. Some of these changes came from the render view template, and for now, I just made sure the themeVersion is correct there (when I add a new theme, will correct differences in fonts file).
• I added docs/theme.md even though the base isn't present as it still seems valid
• The startup template was never reverted from 2.x, and so it doesn't do anything, and I just left my changes on top.
• The header menu is still present as it was recently introduced even though I don't think it does anything - similarly, I just left my changes on top

Issues Resolved

N/A

Screenshot

N/A

Testing the changes

I was able to switch between themes locally, validated correct css files load. Pages look correct and no errors.

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

[codecov]

Codecov Report

Attention: Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.

Project coverage is 63.82%. Comparing base (b3078c4) to head (9e44be1).
Report is 6 commits behind head on 2.x.

Files	Patch %	Lines
src/legacy/ui/ui_render/ui_render_mixin.js	0.00%	3 Missing ⚠️
packages/osd-ui-shared-deps/theme.ts	60.00%	0 Missing and 2 partials ⚠️
src/core/public/ui_settings/ui_settings_client.ts	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #7746      +/-   ##
==========================================
+ Coverage   63.81%   63.82%   +0.01%     
==========================================
  Files        3646     3647       +1     
  Lines       80751    80781      +30     
  Branches    12842    12844       +2     
==========================================
+ Hits        51528    51556      +28     
- Misses      26067    26070       +3     
+ Partials     3156     3155       -1     

Flag	Coverage Δ
Linux_1	29.91% <63.15%> (+0.03%)	⬆️
Linux_2	55.98% <88.88%> (+0.04%)	⬆️
Linux_3	40.54% <60.00%> (+0.02%)	⬆️
Linux_4	31.49% <63.15%> (+0.02%)	⬆️
Windows_1	29.93% <63.15%> (+0.03%)	⬆️
Windows_2	55.93% <88.88%> (+0.04%)	⬆️
Windows_3	40.54% <60.00%> (+0.02%)	⬆️
Windows_4	31.49% <63.15%> (+0.02%)	⬆️

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