add defaultTheme prop to specify default theme if no previous value stored

[Yahkob]

I'm not sure if this is too big of a change but would love to have a way to provide an optional prop to show GraphiQL in a specific theme.

[changeset-bot]

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Latest commit: 4299163

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[linux-foundation-easycla]

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• ✅ login: Yahkob / name: Jake Harclerode (4299163)

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.78%. Comparing base (fa0e7f7) to head (4299163).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3619      +/-   ##
==========================================
- Coverage   60.80%   60.78%   -0.03%     
==========================================
  Files         120      120              
  Lines        5616     5618       +2     
  Branches     1487     1488       +1     
==========================================
  Hits         3415     3415              
- Misses       1749     1750       +1     
- Partials      452      453       +1     

Files	Coverage Δ
packages/graphiql/src/components/GraphiQL.tsx	68.70% <100.00%> (ø)
packages/graphiql-react/src/theme.ts	37.50% <0.00%> (-3.41%)	⬇️