The error looks like the one shown below (Taken from Microsoft Tech Community)

To fix this error, the easiest method as of March 2020 is do the following.

1. Click on “Classic change the look options” menu item shown in the screenshot above.
2. Choose one of the theme.
3. Preview it by clicking on “Try it out”.
4. Then click on “Yes, keep it”
5. Again load the home page
6. Try changing the theme, and the error should be gone.

Thanks to Rob for providing the work around here.

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