Make the dark-mode selected timeline background more similar to non-selected color/contrast, and use slightly brigher colors for the timeline and category

[arai-a]

Fixes #5822

This does:

• Construct the selected timeline color based on the non-selected timeline color (--raised-background-color), and the light mode's selected timeline color (#edf6ff), with more reduced brightness, to make the graph colors more prominent
• Increase the brightness of the following timeline/category colors:
  • purple (also increased the alpha for unselected style)
  • green
  • yellow (also increased the alpha for unselected style)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.57%. Comparing base (4245ceb) to head (25697af).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5823   +/-   ##
=======================================
  Coverage   85.57%   85.57%           
=======================================
  Files         320      320           
  Lines       31446    31448    +2     
  Branches     8656     8666   +10     
=======================================
+ Hits        26911    26913    +2     
  Misses       4104     4104           
  Partials      431      431           

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[mstange]

Thanks!