Update readme for theme usage

[shubham-eaton]

Changes proposed in this Pull Request:

• Added readme for theme usage

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.58%. Comparing base (978cee8) to head (4dbfdcf).
Report is 2 commits behind head on release/mui-v6.

Additional details and impacted files

@@                Coverage Diff                 @@
##           release/mui-v6     #674      +/-   ##
==================================================
- Coverage           72.67%   72.58%   -0.09%     
==================================================
  Files                  89       89              
  Lines                1127     1127              
  Branches              246      246              
==================================================
- Hits                  819      818       -1     
  Misses                131      131              
- Partials              177      178       +1     

Flag	Coverage Δ
unittests	72.58% <ø> (-0.09%)	⬇️

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