Support for theme customization using CSS variables

[hnagrath09]

I tried creating a separate theme.css file, but tailwind.config.js file was unable to pick values of defined CSS variables in theme.css file. To make it work, I have created CSS variables in index.css file.

• Added css variables in index.css file of tail-kit
• Extended theme in tailwind.config.js of tail-kit
• Updated test cases with new classes

[codecov]

Codecov Report

Merging #60 (076a3bf) into develop (b425bfa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #60   +/-   ##
========================================
  Coverage    95.10%   95.10%           
========================================
  Files           46       46           
  Lines         1307     1307           
  Branches       556      556           
========================================
  Hits          1243     1243           
  Misses          60       60           
  Partials         4        4           

Impacted Files	Coverage Δ
packages/tail-kit/src/components/button/button.tsx	100.00% <100.00%> (ø)

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[github-actions]

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