Please add this code to the Custom CSS field in the Content Views >> Settings page:
/* Show borders around each post in Grid */
.pt-cv-ifield {border: 1px solid #eee; padding: 15px;}
That will work for all grids.
To show the border for a specific view only, please use this CSS instead:
#pt-cv-view-VIEW_ID .pt-cv-ifield {border: 1px solid #eee; padding: 15px;}
(and replace VIEW_ID with ID of your View)
Best regards,