This is a good question. There are a few ways to look at this. So, I am sure that there will be some differences of opinion. Let me answer this question by making a few points.
First, there is some overlap between theme and subject. So, a subject matter and theme can be the same thing but this is not always the case. Let me give you an example, if there is a love story and main point of the story is about true love, then the theme and subject would be on love. There is overlap.
Second, subject and theme can also be different. The subject is the broader topic and the theme is a variation of the subject matter. The best way to underline this is by another example. Let's take the book, A Christmas Carol, as an example. The subject matter of this book is Christmas, but the theme is on giving and being generous.
In light of these two points, think of subject as the general topic and the theme as the specific topic.
Subject matter is what something is about. The theme is the message that the author of this subject matter wishes to convey. For instance, Edward Arlington Robinson's well-known poem "Richard Cory" is about a wealthy man, who for all appearances, is a fortunate man: he is wealthy; he is handsome and well-dressed; he has good manners--"he glittered when he walked"--and made others envy him. Yet one night, Cory killed himself. This all is the subject matter.The theme, however, differs from the subject matter. The theme is that appearances can deceive, and that people may not always be what others believe them to be. Another theme may be that the wealthy are often lonely and in despair.
In another example, John Steinbeck's novel The Grapes of Wrath has as its subject matter the displacement of the sharecroppers of Oklahoma after the Dust Bowl and during the Great Depression. The Joad family must load all their belongings onto an old truck and drive like so many others, staying in tent camps, working for a pittance, and hoping to find work. The themes, however, are the strength of family, the endurance of man and man's inhumanity to man. The Joads and others learn that they must rely upon others and form a brotherhood of men.
And some day--the armies of bitterness will all be going the same way. And they'll all walk together, and there'll be a dead terror from it.
And, Tom Joad says,
"Well, maybe like Casy says, a fella ain't got a soul of his own, but on'y a piece of a big one--"
Subject can be related to topic while theme will always be, to some extent, conceptual. Subject matter can be topical, localized and stated often as a simple noun phrase. (Examples: the medical profession or the hardships of life as a performing artist) Theme can also be described through simple noun phrases, but is often best stated as an issue rather than as a static or inert fact.
Theme grows out of the dynamics of a text and relates to the commentary a work is undertaking, explicitly or implicitly. Thus "the nature of performed identity," "the emotional bankruptcy of superficial desire" and "the material corruption of the ideal American dream" are all themes in The Great Gatsby, whereas the subject of that novel can be said more simply to be "life in the upper classes during the Jazz Age."
We can see here that the subject may be made up of an assortment of facts while the themes address a deeper issue that arises from the subject matter. It is tempting to offer an analogy and suggest that subject is the canvas and theme is what emerges upon that canvas through the artistry of the author.
Additional examples may help to further clarify the differences between subject and theme.
Looking at Robert Frost's famous poem, "The Road Not Taken," we can point to ways that subject matter can be used to generate and express theme while remaining almost entirely separate from the themes of the work (and therefore figurative in its usage).
In drawing a contrast between subject and theme, we might argue that the subject of the poem is a specific situation wherein a man comes to a place where he must decide what path to take through the woods. The poem describes the woods and the appearance of each path. The appearance of the paths and of the woods, however, are not necessarily descriptive of the poem's theme(s) and instead constitute the subject matter of the poem.
Another example that may be helpful is George Orwell's Animal Farm. While the overt subject matter relates to a farm run by animals, the themes of the text have nothing to do with horses and pigs and everything to do with the politics of economic philosophy and economic systems, the corrupting influence of power and the importance of education in regards to citizenry in functioning societies.
Again we see that the contents of the story as far as what is actually discussed on the page (in this case a farm run by animals) constitutes the subject of the work while the implications of the work can be seen to be almost separate from that subject matter (because the subject has been utilized figuratively -- used to create a meaningful metaphor, analogy or allegory).
To be clear, the subject in each of these cases is used to express the themes of the work. The subject is not separate from the theme. Rather the subject is antecedent or constituent to the theme. The themes of these works are contingent to the subject, derived from it and so only comes into being as a conceptual outgrowth of the subject.
To ascertain the theme of a work, one might ask what is the author saying with or through the subject matter of the work?
your answer is awesome! thank you very much.