Fix JupyterHub Config Theme Retrieval

[jbouder]

Reference Issues or PRs

N/A

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Any other comments?

• Updates to get theme from JupyterHub config template_vars

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Name	Status	Preview	Comments	Updated (UTC)
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