refactor(widgets): 💡 use variant_color in the class of Slider

[M-Adoo]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.64%. Comparing base (a7d939d) to head (99ac4a4).
Report is 48 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
- Coverage   83.93%   83.64%   -0.29%     
==========================================
  Files         200      209       +9     
  Lines       26350    28449    +2099     
==========================================
+ Hits        22116    23796    +1680     
- Misses       4234     4653     +419     

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