One theme could be the pride of the narrator. He is so concerned about out-smarting his friends that he, in effect, out-smarts himself, by moving his bed to the middle of the floor and making his room dark. When the valet trips over him and spills his breakfast, he is not being “pranked” – the valet simply could not see him and did not expect to find a mattress in the middle of the room. So in a sense the “prank” is the narrator’s own suspicious nature!
Another possible theme could be appearance vs. reality: the narrator is paying close attention to the behavior of his friends, and his interpretation of their good spirits is that they must be up to something. The reality of the situation is not really available to us as readers; we can assume, however, by the events of the story that the narrator was wrong about his friends.
A third possible theme is paranoia. If we think about the narrator as someone who is paranoid, then his actions and suspicions take on the character of mental illness: everyone is out to get him; even though he will go to any length to avoid being pranked, jokers are the only sort of people he cares to know; his suspicions cause him to behave in an irrational manner (by moving the bed and assaulting the valet). Even the final line of the story (“How they all laughed that day!”) suggests that despite all his efforts he has been “gotten” in the end.
Finally, another theme could be victimization. The tone of the story is very light-hearted, but there is an edge to it – a sense that perhaps not everything is as happy as the narrator would have us believe. We don’t have much information about the relationship of the narrator to these “friends,” but there does seem to be an abusive element to it. Another aspect to this theme would be the idea that the narrator is self-victimizing; why else would he prefer to associate only with practical jokers, unless he somehow enjoyed or desired to be the object of such jokes? In that case, the end of the story, and his reflection on his friend’s laughter, could be an expression of gratification: their laughter affirms his value as a suitable “target” for their pranks.