test(theme): add tests for mergeWithDefaultTheme

[markov00]

fix #59

It also fix 2 possible mutations on the default theme

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Proper documentation or storybook story was added for features that require explanation or tutorials
• Unit tests were updated or added to match the most common scenarios
• Each commit follows the convention

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 2.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   73.62%   75.82%   +2.19%     
==========================================
  Files          28       29       +1     
  Lines        1183     1183              
  Branches      217      215       -2     
==========================================
+ Hits          871      897      +26     
+ Misses        297      271      -26     
  Partials       15       15

Impacted Files	Coverage Δ
src/lib/themes/theme.ts	100% <100%> (+89.65%)	⬆️
src/lib/themes/dark_theme.ts	100% <0%> (ø)

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[emmacunningham]

Ran test locally & code LGTM (hooray for increasing test coverage!)

[markov00]

🎉 This PR is included in version 2.1.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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