Theme UI styles are not applied to MDX content

[maiertech]

This is a regression. Theme UI's styles object is not applied during MDX transformations.

There is one ThemeProvider on the page that receives the correct theme prop with correct styles object. styles object should be automatically applied during MDX transformations, but it is not.

There are two nested MDXProviders on the page. The first one contains a non-empty components prop, the second one contains an empty components prop. Not clear where the second one is coming from.

[maiertech]

Underlying issue: system-ui/theme-ui#1345.