In the current main branch we can see in Reboot > Horizontal rules that all borders have the same thickness, event with .border-2 and .border-3.
This PR proposes to add .border for .border-3 and .border-3 to be actually applied.
It also means that we can't use anymore .text-danger but replace it by .border-danger due to the use of CSS vars.
I believe that the description associated with this example is still accurate but I'd appreciate a double-check.