[docs] `theme.css` in CSS Modules demos are linked incorrectly in CSB

[mj12albert]

Example: https://master--base-ui.netlify.app/components/react-collapsible

For the first demo, choose CSS Modules and click the CodeSandbox button. The theme.css file is generated correctly, but not linked in index.html:

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[mj12albert]

@vladmoroz Is this an issue? The sandboxes generated off the /new docs route seem fine

[vladmoroz]

If /new works then it's fine (not sure why it'd work, but whatever)
We'll move over all from /new to root before the release

[github-actions]

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