Fix erroneous definition for --mjx-bg1-color.

[dpvc]

This PR fixes an incorrect CSS setting for the --mjx-bg1-color. It is usually replaced when MathJax initializes itself, but the initial value should really be made correct anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (5caee44) to head (a4b9da1).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1438   +/-   ##
========================================
  Coverage    86.49%   86.49%           
========================================
  Files          340      340           
  Lines        86044    86044           
  Branches      4833     4833           
========================================
  Hits         74425    74425           
  Misses       11595    11595           
  Partials        24       24           

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