adwaita.css: Apply new style to path-bar DnD Targets
path-bar DnD targets were using the 'green border' style when targeted. This style is inconsistent with the 'solid bottom border' style of the other path-bar actions.
This commit updates the target style to show a 'solid bottom border' in the theme's drop target color.
Fixes #765 (closed)
Edited by Timothy OBrien