Add flag to conditionally render MUI theme

[jenny-s51]

Description

Re-applies switch to MR UI in order to conditionally toggle MUI theme on / off.

How Has This Been Tested?

Toggle switch in page masthead and verify that the MUI theme is toggled on / off based on the value of the switch.

Tests N/A, visual updates only.

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
• Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[google-oss-prow]

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