Bootstrap borders are quite light compared to default

[KorvinSzanto]

Compare the two:

Concrete default	Bootstrap default
$border-color-translucent: rgba(#000, 0.075);	$border-color-translucent: rgba($black, .175) !default;

Ours is so light that the borders are hard to distinguish, at first I was thinking my display had settings misconfigured or something.

We should increase the darkness, or maybe use a different color so that our borders are distinguished against the drop shadow.

[aembler]

This is generally done by design – bootstrap is many things but I'm not sure I'd call its design out of the box very attractive. Years ago we experimented with forms that had no fields (per Franz's interest) (similar to material design at the time.) While that was too over the top, the goal was to make our UI less "chunky". I'm not sure that making all the borders more prominent accomplishes that.

[KorvinSzanto]

Bootstrap tries to work within their accessibility requirements, and contrast is a big part of that.

It'd be better to tweak borders at a more granular level, like for form-controls, and not globally for everything with a semitransparent border like we have today.