fix(styling): flatpickr bg-color should be using SASS variable

[ghiscoding]

No description provided.

[codecov]

Codecov Report

Merging #616 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #616   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          149       149           
  Lines         9974      9974           
  Branches      3528      3528           
=========================================
  Hits          9974      9974           

Impacted Files	Coverage Δ
...dules/angular-slickgrid/filters/dateRangeFilter.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 07bd99a...10a2764. Read the comment docs.