Although the two are clearly meant to be the subject of the work, they are depicted as being relatively small in scale - the surrounding porch, walls and lawn seem to nearly overwhelm them.
This is a common theme in Hopper's works, which typically feature individuals in a state of somewhat alienated isolation - however, most of those works are much more metropolitan in nature.
This piece is in stark contrast to the setting of those other pieces, but still perpetuates the theme despite the intimate setting. The couple seems very vulnerable, as if there was something threatening waiting for them in the darkness.
They also aren't looking at each other - while the man is looking down, his gaze is still somewhat on the woman, her eyes are trained on the floor. This speaks to a lack of connection, even in a situation in which they should be the focus of each other's attention.
The colour scheme also contributes to this detached look. The white facades of the house would look cheery and wholesome by day, but become eerily clinical when framed by the darkness of the night.
The couple, on the other hand, are clothed in soft colours that seem almost childlike in the way in which they suggest stereotypical gender roles. The pink and blue call attention to the fact that these people are very young, and are likely ill-prepared for the challenges of the world at large.
While they may be able to enjoy a tranquil moment now, the 'real world' looms large over them, and even this kind of tenuous connection might soon be beyond them.
It seems inevitable that they will eventually become just as lost in the world as Hopper's conventional subjects typically are.