Our default border color is defined by the $color-lighter variable, and you could update this by creating a new System Property called "css.$color-lighter" with the Value set to "#030303". That said, this variable is used for content other than Border colors, so you'll see this change other elements as well (left nav Applications, Edge menu, List and Form headers, lock and search icons).
Beyond that, I'm not sure if there's going to be a good way to define this globally. You could use field styles to apply the color on a one off basis, or style this in a CMS as desired, but there's not an overall setting for just changing the border color of all input fields.
