fix: Align active subnav item "active" indicator to overlap subnav border

A fix for the issue described in this ticket - #25 (closed) - in which the bottom border on active .Subnav elements should overlap with the bottom border on the .Subnav, rather than stack on top of it. This is achieved by applying a negative margin-bottom value on .Subnav ul items the same height as .Subnav's bottom border.

Closes: #25 (closed)