theme code simplification

[indirectlylit]

Summary

Follow-up from #8805 with more to come after.

In the theme code I originally tried defining constants in the Python theme_hook module which are simply references to strings used elsewhere in the code. These constants turn out to be more trouble than they're worth, mainly because they add a layer of indirection when someone is trying to figure out how all the theme logic fits together. They fail the grep test.

Additional changes:

• remove unnecessary checks
• simplify default theme

References

Will link to this PR from other relevant repos and PRs. See back-references below description.

Reviewer guidance

Seem reasonable?

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

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Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[indirectlylit]

@rtibbles this is ready for review.

I'm working on a follow-up PR which adds theme spec validation to this based on #8640 and #8878 and removes validation from Python

[indirectlylit]

These changes should be fairly low-risk, but there is still a non-zero risk of regression. If we're down to the wire on 0.15, recommend combining with #8902 and putting them both in a patch of 0.15

[indirectlylit]

merging into theme-updates feature branch

ref: #8918