Fix menu styling on JupyterHub 5.0, require 5.0

[krassowski]

Reference Issues or PRs

Fixes #20

Before	After

What does this implement/fix?

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• 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?

[krassowski]

Looking at an older deployment I see that the top bar used to be dark prior to JHub 5 and that now the nebari logo text is not visible. Before:

[krassowski]

OK, looking good now I believe:

[marcelovilla]

Thanks @krassowski !