[dev-overlay] sync horizontal scrollbar style (#77769)
Following up on https://github.com/vercel/next.js/pull/76953.
In macOS, the scrollbar color is still light in dark mode. So, we
modified the style for it at
https://github.com/vercel/next.js/pull/76509. However, it didn't cover
the scrollbars of the children of the dialog. Therefore, we sync the
scrollbar of children to be dark in dark mode as well.
| Before | After |
|--------|--------|
| 
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