When starting a new presentation, you have a choice to choose a “theme” in the software presentation. Most of these are actually just backgrounds. They may be stylistic, but don’t carry a consistent idea through the entire presentation.
Some of the best examples of a true theme (and a source of presentation envy for me) are in presentations by one of my colleagues and an up-and-coming star in the world of EM and Ethics, Dr. Nathan Allen. Here is an example of one of his presentations: