Use darker form field borders

[kjellr]

Closes #17199.

This PR changes our default form field border color from $dark-gray-150 to $dark-gray-200. It's very minor, but ensures more accessible contrast ratios when these borders appear against gray backgrounds in the rest of WP-Admin. More details in this trac comment.

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[jasmussen]

Sure.

Have you checked it applies to anything resembling an input field? Searchboxes, dropdowns, checkbox borders?

[kjellr]

Yep, it seems to be properly applied to select fields, as well as number fields:

It's applied to the search field too, via 67a9d6e.

Checkboxes and toggle controls actually use $dark-gray-300 already, so those should already pass. I could make them lighter to match, but that seems like a bad move. 😄

I'll hold off on merging until I get a ✅ from the a11y team.

[enriquesanchez]

This looks like a good improvement! It's great that you're also applying this treatment to other input elements 👍

[kjellr]

Thanks, folks!