Use the new .table-group-divider to create your own dividers as desired. Would love to find a better way to handle border-color for this, but for now, this is at least opt-in.
I've applied it by default in another way for our docs tables to help differentiate our content vs our components.
Fixes #35342 (closed).
Also you'll notice the new calc() function, which is in preparation for more CSS vars here.