Restore slightly darker default border-color

[mdo]

Because people have eyes and would really like to see things lol. Saving for v5.4.0 though so it's not a surprise for folks.

Fixes #38480, fixes #41414.

[julien-deramond]

Agreed — this change should land only in v5.4.

The new value definitely improves contrast against the white background. Approving it as-is, though I think that for form controls (like input borders, checkboxes, etc.) — and possibly other components — the color could be even darker to achieve a 3:1 contrast ratio with the white background, instead of 1.49 with this new color. 🫣

Adding Patrick as a reviewer to get his feedback as well, either for this version or potentially for a more contrasted color for v6.

[patrickhlauke]

The new value definitely improves contrast against the white background. Approving it as-is, though I think that for form controls (like input borders, checkboxes, etc.) — and possibly other components — the color could be even darker to achieve a 3:1 contrast ratio with the white background, instead of 1.49 with this new color. 🫣

Adding Patrick as a reviewer to get his feedback as well, either for this version or potentially for a more contrasted color for v6.

Agree we can merge this now, but would be good to darken a tad more to hit 3:1 if possible