Extract font color of month and time headers to vars

[firec0der]

This tiny patch will give us the opportunity to set font color of these headers.

[codecov]

Codecov Report

Merging #1090 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1090   +/-   ##
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  Coverage   84.13%   84.13%           
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  Files          13       13           
  Lines         876      876           
  Branches      148      148           
=======================================
  Hits          737      737           
  Misses         35       35           
  Partials      104      104

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[firec0der]

Btw, I mentioned that there is little mess with these variables.
There is no way to customize only the color of "prev" and "next" arrows. There is $datepicker__muted-color variable for it, but it's also used for disabled dates.

I can separate it if you want.