Themes: fix shellcheck (SC2154); Part A

[gaelicWizard]

Please review commit notes as appropriate.

Description

This is a chunk of my SC2154 branch (#1954), hopefully small enough to merge quickly.

Motivation and Context

I think the changes here should be uncontroversial and without behavior changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[gaelicWizard]

@davidpfarrell, I hope this PR is short enough to review in one go. I'm happy to pull any file that doesn't quite fit to get the rest in. It's just really hard to keep track of all these branches. Thanks!

[NoahGorny]

LGTM

[gaelicWizard]

Anything else needed on this PR? I've got too many open branches and I'm hoping to close out a few 😃