Define theme vars in terms of AntD vars

[yurishkuro]

An even cleaner approach where we don't define custom colors in vars.css twice, but instead rely on AntD theme support which already defines over 300 --ant-xxx-xxx variables (captured in docs/adr/0001/antd-vars.css added in this PR).

This actually makes the dark theme a lot more usable because by default we're applying colors that AntD already transformed for a dark theme. Of course there are some glitches, e.g. we need to decide what the navbar should look like the dark mode.

For comparison, the website now supports dark theme too an its navbar doesn't change between themes

It would actually be great if we aligned the actual Jaeger UI with the website look a feel at least on that level.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (9c75545) to head (11ae460).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3245   +/-   ##
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  Coverage   97.83%   97.83%           
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  Files         261      261           
  Lines        8209     8209           
  Branches     2178     2178           
=======================================
  Hits         8031     8031           
  Misses        175      175           
  Partials        3        3           

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