A Theme is certain aspects of given media that are closely similar to one another in a number of ways. The criteria for something to be considered a theme is various and it includes things such as setting, characters, context, etc. As long as two pieces of material have at least one of these criteria in common or similar they can be considered a part of a theme.
The Grand Theme
Of course the Grand Theme of the Parody Universe is that every piece of material within it is a Parody of some form, however there are some limits that prevent this definition from spreading ever wider into vagueity, meaning there is not a "Grander Theme" from the perspective of the Parody Universe, such as they are all media or some basic state of merely existing to do so would render any meaning to the word "Theme" useless.
Examples in the Parody Universe
As the Parody Universe continues to expand, the number of examples of themes in parodies grow just as rapidly, however here is a brief summation on the kinds of themes commonly in use.
These are parodies that have a theme in respect to having the same or close to the same set of characters. The standard Hitler Parodies as well as Stalin Parodies, Franco Parodies, Gaddafi Parodies, the American Girl parodies and Ceaușescu Parodies.
These are parodies that have the same premise such as Hitler is Trapped or Hitler Interviews
It is possible for parodies to have multiple themes, and also beyond the "Grand Theme" it is possible for parodies to have no theme, it all depends on the parody itself and other parodies that are used in comparison.