use background-default token for gl-new-card background

cf. #457239 (comment 1929360177)

(Note that I've labelled this as grouptenant scale because a group is required and this card is very frequently used in project settings and admin, but this work is only very loosely associated with that group if at all. This is not group-specific work and the owners of this are, if anything, teamFoundations ?(not sure if that's even the right label for them))