fix(theme): fix issue that mui was loaded multiple times

[christoph-jerolimov]

Description

WIP

Which issue(s) does this PR fix

TODO

PR acceptance criteria

Please make sure that the following steps are complete:

• GitHub Actions are completed and successful
• Unit Tests are updated and passing
• E2E Tests are updated and passing
• Documentation is updated if necessary (requirement for new features)
• Add a screenshot if the change is UX/UI related

How to test changes / Special notes to the reviewer

[openshift-ci]

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[openshift-ci]

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[github-actions]

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[github-actions]

The image is available at:

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• quay.io/janus-idp/backstage-showcase:pr-2130-310381c7

[github-actions]

The image is available at:

• quay.io/janus-idp/backstage-showcase:pr-2130 or
• quay.io/janus-idp/backstage-showcase:pr-2130-30b4842d

[openshift-merge-robot]

PR needs rebase.

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