“Areas” vs. “Themes” in the classification of a university’s topics
Many departments will organize themselves internally in multiple various different ways, base on how they've organized their program.  For example, where I went the Electrical Engineering and Computer Science Department was organized in at least the following (partially independent) ways:


Three different degree tracks (EE, CS, and EECS): which one you chose dictated your graduation requirements.
Seven focus areas (devices, signal processing, electrodynamics, AI, algorithms, systems, biomedical): these were used in organizing graduation requirements.
Several major "laboratories" comprising up to 100 faculty or more, which were primarily EECS but also overlapped with other departments: these were the primary organizations of research and graduate life (beyond one's own supervisor, of course)
A number of "themes" within the focus areas, that were used for determining aspects of Ph.D. qualification.


Other departments are likely to have similar degrees of intricacy, as the result of the evolution of complex negotiations between different people and organizations over many decades.  Moreover, they are likely to use the same words differently, because there are only so many ways to say things like "area" and "theme" and lots of different odd organizational specifics that one might be mapping them to.

In your own case, it sounds like the "areas" are more durable aspects of study, where the "themes" are current hot topics.  The only way to know for certain what they mean to that organization, however, is to read the university's documentation online (if they are well organized) or to contact the people there directly.