Add dark mode CSS, improve accessibility

[machawk1]

Adds an initial set of style for the replay UI to be adaptive to
dark mode. This also moves to use the version of the logo with
the light border that is only apparent in dark mode. The kerning,
coincidentally, is better in this version than the version that
has been used in the replay UI.

Some other tweaks have also been applied to improve the
accessibility including some markup additions and adjustments.

This PR consists of just some initial style for dark mode, and
could use some further tweaking, but it should meet the requirements
of #695 in making the UI slightly adaptive.

Closes #695

[machawk1]

For reference, some screenshot that, again, can tweaked.

[codecov]

Codecov Report

Merging #708 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #708   +/-   ##
=======================================
  Coverage   29.24%   29.24%           
=======================================
  Files          10       10           
  Lines        1248     1248           
  Branches      185      185           
=======================================
  Hits          365      365           
  Misses        860      860           
  Partials       23       23           

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[machawk1]

One other adjustment while I was tweaking CSS, the "URIs" abbreviation underline is broken per the default browser style and the U apparently being a descender. I tweaked this in b6d33f2.

Before:

After (per b6d33f2):

[ibnesayeed]

The CSS file does not have a trailing new line, other than that LGTM!