When theme becomes overt it may dominate the art and the result can be more like documentary or at worst, propaganda. Serious plays and true comedy are about something that matters, but much of film and television is merely about what happens next.
Theme arises from moral, social, and political values that are personal to the author.
Most writers don't intentionally plant themes in their work. Since they are so deeply embedded in their personalities, themes just flow naturally into the script. This is why certain themes tend to reappear in many works by the same author. Like plot structure, themes are seldom singular. That is, there are several themes within all works although one may be dominant.