Thematology

It is the contrastive study of themes in different literary texts. As a subfield in comparative literature according to the French school, thematology is a comparative study of literary works as they relate to other literary works beyond their national borders as far as the themes are concerned. It is a more specific and limited investigation in the field of comparatism.

A literary text has two main components i.e. form and content.
The form is about the language and the structure. It is how the content is presented. A specific form may help putting a specific content into value. For instance, Alice Walker's The Color purple is a novel about an oppressed and silenced girl. Walker used the epistolary form (letter novel). Celie addresses letters to God. It is more likely for a silenced girl to write letters for their private and discrete nature instead of being a traditional narrator protagonist in a novel. Also because Celie is almost illiterate, her letters are written in the dialect of blacks in the south instead of standard English and the epistolary form    meets the purpose of that informal situation.

The content is what the literary text is about.
The content is either derived from reality. The text may be drawn or inspired from a given reality. It may reflect the reality of a specific time and a specific place as it may reflect a more universal reality that is related to human nature and existence. Also, it may be concrete in that it reflects concrete situations of human condition as it may be abstract in the form of philosophical reflections, meditations and commentaries about a specific or a universal reality.

Or it may be derived from the folklore, the oral tradition and the collective imagination of a specific group. Homer's The Odyssey and The Iliad bear a content that is largely inspired and drawn from Greek mythology.

Terminology: 

Theme vs Subject matter: The subject matter is larger and wider in scope in comparison to the theme. The theme is more specific. For instance, "love" is the subject matter in Fitzgerald's The Great Gatsby. The theme is more precise. It is love in the age of materialism.  

Motif vs Theme: The theme is the central idea or message where as the motif constitutes the unit from which the theme is built. The motif could be an image, a sound, an action, an object, a character, a literary device, a word, a phrase… with a symbolic significance that helps constructing and consolidating the central idea or the theme. It contributes towards the development of the theme. A motif is a recurrent idea or symbol or image that develops and explains the theme.
The theme of "the corruption of the American dream" in The Great Gatsby is built up and consolidated by the succession of certain motifs like the green light at the end of Daisy's Dock. Green the color of the American shores as first perceived by the Dutch sailors who came to America to fulfill their dreams is also the color of dollars (materialism). Another motif is the movement east instead of west. Most characters moved east to fulfill their dreams as young people. It is the opposite direction of the American dream. This explains why the narrator Nick Caraway decided to return to the midwest at the end of the novel. the dichotomy east/west is a recurrent motif in the novel that serves the purpose of consolidating the theme of "the corruption of the American dream"

Symbol vs Motif: There is a difference between a symbol and a motif in that the symbol is an image, idea, sound, or words that represent something else and help understanding a given idea or a thing where as a motif is an image, idea, sound, or word that help understand the central idea in the literary work. Another difference is that the motif is recurrent where as the symbol may appear once or twice in a literary work.

Leitmotif vs Motif: the literal translation of leitmotif (a german word) is leading or guiding motif. However the real meaning of the concept is different. A leitmotif is more easily noticeable in opera and cinema where a specific melody is associated to character or a given situation or a given setting. Examples from literature: In Shakespeare's Macbeth, thunder and lightning are associated with the supernatural world of the witches. In Ngugi's A Grain of Wheat, water imagery is always associated with Mugo. In The Great Gatsby, white is always associated with Daisy or again the word "voice".