Some will put forth that theme is the premise of a story that illustrates the results of certain kinds of behavior. Clearly each of these might show up in the very same story, and each has a somewhat thematic feel to it. But just as certainly, none of them feels complete by itself. This is because each is just a different angle on what theme really is. In fact, theme is perspective.
Perspective is relationship.
Theme describes the relationship between what is being looked at and from where it is being seen. This is why theme has traditionally been so hard to describe. It is not an independent thing like plot or character, but is a relationship between plot and character. As a familiar example, think of the old adage about three blind men trying to describe an elephant. Each is like a character in a story, and their investigation of the beast is like the plot.