Change diff type for Themed Mixin to auto

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Use auto over shallow for the diff type of theme, classes and extraClasses in the Themed mixin to prevent unnecessarily invalidating when undefined is explicitly passed to a widget.

Resolves #609

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[codecov]

Codecov Report

Merging #685 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #685      +/-   ##
==========================================
- Coverage   97.77%   97.77%   -0.01%     
==========================================
  Files         117      117              
  Lines        6615     6614       -1     
  Branches     1502     1502              
==========================================
- Hits         6468     6467       -1     
  Misses        147      147              

Impacted Files	Coverage Δ
src/core/mixins/Themed.ts	100.00% <ø> (ø)

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