Replace rem size literals with constant (#11272)

This PR replaces the rem size literals with the `BASE_REM_SIZE_IN_PX`
constant.

This way we can retain our source of truth for the rem size that we're
designing against.

Release Notes:

- N/A
4f5312804dMarshall Bowers committed on 5/1/2024, 9:46:43 PM· committed by GitHubparenta194c0a
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