Add dark mode versions of the fullscreen icons

[canova]

Fixes #5970.

These svg icons were hardcoded to black color, which made them very difficult to see in the dark mode. This commit adds dark mode equivalents of these icons to make sure that they are visible there as well.

Deploy preview

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (c155fdf) to head (920e7c9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5972   +/-   ##
=======================================
  Coverage   85.33%   85.33%           
=======================================
  Files         323      323           
  Lines       32262    32262           
  Branches     8895     8810   -85     
=======================================
  Hits        27532    27532           
  Misses       4298     4298           
  Partials      432      432           

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[fatadel]

Would be nice to see a deploy preview profile :)

[canova]

Here it is

[fatadel]

I wonder why fullscreen and fullscreen-exit are of different sizes but that does not seem to be introduced here... Thanks for the PR!

[canova]

Thanks! Yeah, that's weird but they are directly from: https://acorn.firefox.com/latest/desktop/styles/iconography-OJ7DtlvA It looks like the upstream's svg file is incorrect (but not a big deal).