Implement dark mode

[arai-a]

Fixed #2097

This patch stack implements the dark mode:

• almost all colors and icons are moved to variables
• the variables are defined either in the root, or as the element internal variables
• canvas-related colors are handled in the script side, with a new dark-mode module added

This stack doesn't cover the user documents.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.64%. Comparing base (1a808fe) to head (ab93549).
⚠️ Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/marker-chart/Canvas.tsx	85.18%	4 Missing ⚠️
src/utils/dark-mode.ts	76.47%	4 Missing ⚠️
src/profile-logic/graph-color.ts	0.00%	1 Missing ⚠️
src/utils/colors.ts	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5740      +/-   ##
==========================================
- Coverage   85.64%   85.64%   -0.01%     
==========================================
  Files         313      314       +1     
  Lines       31013    31059      +46     
  Branches     8535     8543       +8     
==========================================
+ Hits        26562    26600      +38     
- Misses       4021     4029       +8     
  Partials      430      430              

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