Do not show theme selector if only one theme is configured

[tvdeyen]

Summary

There is no usecase for selecting from one theme.

Checklist

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[kennyadsl]

Thanks @tvdeyen!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.92%. Comparing base (96baef2) to head (d7ce879).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5705   +/-   ##
=======================================
  Coverage   88.92%   88.92%           
=======================================
  Files         700      700           
  Lines       16653    16653           
=======================================
  Hits        14808    14808           
  Misses       1845     1845           

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[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	v4.3

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details