Thematology is simply the study or science of ("ology") a theme. The purpose of thematology is to study a theme typically in regards to geographical distributed cultures. While this may seem even more confusing than the term thematology, consider the phrase once dissected.
Geographical means relating to geography. Geography is (according to Google):
The study of the physical features of the earth and its atmosphere, and of human activity as it affects and is affected by these.
Distributed means to give out shares of something or to divide between many.
Lastly, cultures means (according to Merriam Webster):
the customary beliefs, social forms, and material traits of a racial, religious, or social group.
Therefore, geographically distributed cultures refers to the study of the whole of the earth in regards to how cultures are affected by their distribution on earth. (How a region survives given it is mountainous or, how a people survive given their resources.)
This being defined, themaology refers to themes which are present regarding collections of works which deal with geographically distributed cultures.