Before we talk about the difference between a marked and an unmarked theme in linguistics, let's take a moment to define a couple terms. First, a clause is a group of words that contains at least a subject and a verb. The following are examples of clauses: “Anthony made supper”; “Because we went home...”; “Who told the secret.” We won't get into types of clauses here, but note that each of these examples has a subject (Anthony, we, who) and a verb (made, went, told).
Second, a theme is the launching point of a clause, the first element(s) of it (at least in English). Writers choose themes for their clauses based on what they most want to emphasize. The rest of the clause is called the rheme. Let's look at some examples.
Here's a clause: “Jennifer went straight home after the movie.” What is the launching point of this clause, the first element, the emphasized component? It's clearly “Jennifer.” That's the theme. But what about if we write the clause like this: “After the movie, Jennifer went straight home”? Now what is specially emphasized? This time the writer focuses on “after the movie,” so that becomes the sentence's theme. Why would the writer do this? Perhaps he wants to give special attention to where Jennifer has been (did someone think she was at a party instead of the movie?).
Now let's turn our attention to the difference between unmarked and marked themes. Think of unmarked themes as “normal” beginnings to a clause, the ones that are most usually emphasized. Unmarked themes are 1. declaratives or subjects that begin a clause; 2. interrogatives that begin a clause; 3. imperatives that begin a clause. Let's look at examples of each of these.
We see an unmarked theme that is a declarative or subject in one of our examples above: “Jennifer went straight home after the movie.” “Jennifer” is the theme. Unmarked interrogatives can look like this: “Why did Jennifer go straight home?” The theme here is “why,” for it launches the clause, and it is unmarked because it is a normal way to do so.
Another example of an unmarked interrogative is the theme of this clause: “Did Jennifer go straight home?” Here the theme is “Did Jennifer,” for it launches the clause and sets the emphasis. Since it does so in a usual way, it is unmarked. Imperatives can also serve as unmarked themes. They can be inclusive or exclusive. Look at these two clauses: “Let's go home” and “Go home!” The themes are, respectively, “let's” (inclusive) and “go” (exclusive), for they launch the clauses in normal ways.
So what about marked themes? They are marked because they are different than usual, often emphatic, and used to draw attention to a part of a clause that is not usually stressed. We've already seen one of these marked themes above: “After the movie, Jennifer went straight home.” The theme “after the movie” is marked, for it starts the clause in a rather unusual way for special emphasis. Here's another example: “Snails, I just ate snails!” The marked theme of this clause is “snails.” We wouldn't normally write a clause like this unless we wanted to give special emphasis to the direct object (which is repeated again later in the clause). Marked themes, then, place emphasis on parts of the clause that wouldn't normally receive it by positioning them at the head of the clause.