The list of notifications in the Notifications Page is grouped by days, and each group is labelled by the date.
Currently those headers (e.g. "18 May 2016" in the example below) have a bottom border:
Given that the notifications in the list already form a visible block, the line seems to just add noise, and should be removed.
The screenshot below shows the list without the lines (i.e., removing the bottom-border property of ".mw-echo-ui-datedNotificationsWidget .mw-echo-ui-subGroupListWidget-header" ):
The additional bottom space that was added below the "Mark group as read" button (".mw-echo-ui-subGroupListWidget-header-row-markAllReadButton"), should be also removed.