Dark theme fixes

[yurishkuro]

Which problem is this PR solving?

• Resolves [Bug]: Dark mode in v2.14.0 is unreadable #3282

Description of the changes

Fix dark theme readability and layout issues in TracePage

• SpanDetail & Sub-components: Replaced hardcoded colors (backgrounds, borders, and text) with theme-aware CSS variables in SpanDetailRow, KeyValuesTable, AccordianLogs, and other detail views to ensure text readability in dark mode.
• Theming: Added centralized syntax highlighting tokens to vars.css for JSON and key-value displays, and updated TraceTimelineViewer to use them.
• Timeline Header:
  • Made the --surface-tertiary background opaque to prevent content from bleeding through the sticky header.
  • Fixed a 1px transparent gap at the top of TimelineHeaderRow using a CSS pseudo-element to mask sub-pixel rounding issues during scrolling.
• Search Bar:
  • Improved contrast for navigation icons (locate, up, down) and the search 'clear' icon in dark theme.
  • Overrode Ant Design's default disabled button styles to ensure they remain visible against dark backgrounds.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.42%. Comparing base (7ea5367) to head (20cb950).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3285   +/-   ##
=======================================
  Coverage   97.42%   97.42%           
=======================================
  Files         266      266           
  Lines        8297     8297           
  Branches     2166     2195   +29     
=======================================
  Hits         8083     8083           
  Misses        211      211           
  Partials        3        3           

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