Fix documentation theme configuration

[kafitzgerald]

PR Summary

Fixes documentation theme configuration by merging the config option dictionaries.

Closes #252

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
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