Adjust the ExampleThemes* page layout

[danilo-leal]

• [x] Use a meaningful title for the pull request. Include the name of the package modified.
• [x] Test the change in your own code.
• [x] Include the URL where we can test the change in the body of your PR.

This pull request:

• [x] Fixes a bug
• [ ] Adds additional features/functionality
• [ ] Updates documentation or example code
• [ ] Other

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Hey y'all 👋 This is not a functional bug per se, but more of a design issue that I noticed while browsing through these ExampleThemes* components/pages on a small viewport: they bleed off of it, get a horizontal scroll, and the background/title get off-center. The video below illustrates all of this better + the fix (which was just adding wrap to the Flex parent containers):

https://github.com/radix-ui/website/assets/67129314/d7f18215-7906-4109-87a2-f7da494286db