styles: Fix SimpleBar hover styling for simplebar@5.1.0.
styles: Fix SimpleBar hover styling for simplebar@5.1.0.

Follow an upstream adjustment to the styling of the vertical
scrollbar (but not the horizontal scrollbar).

https://github.com/Grsmto/simplebar/issues/420

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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