A "theme" is simply what the clause is about. In the simplest kind of English sentence, it is the subject, which generally comes first. For example, in the sentence "Mary sings," Mary is the subject (and the theme), and "sings" is the rest of the clause, or the rheme.
If the theme is the same as the subject (if, in other words, it is mapped onto the subject) in a declarative clause, the theme is unmarked. The subject is usually the theme, unless there is a reason for another element of the clause to be the theme. If the theme in a declarative clause is something other than the subject, it is a marked clause. "Marked" in this sense means that the theme is less frequent or is more unusual. For example, a marked theme could be a prepositional phrase, a complement (which could function as the subject but is not), or an adverbial phrase. An example is the clause, "An easy task it is not!" In this example, "an easy task" is the marked theme. Another example is "Her friend she loves to drive around." In this case, "her friend" is the marked theme, as it is the complement in the clause.
Language structure can be very complex and confusing. When discussing marked theme and unmarked theme we are talking about clauses. A clause is a grammatical unit,which constitutes a group of words consisting of a subject and a verb. Theme is always what comes first in an English clause. It provides the context of the words. A marked theme tells one directly what a clause is about.
A marked theme has an initial presentation such as "We could eat those berries!" An unmarked them would not have the we in it and would be presented as "Could eat those berries." An unmarked theme consists of the process only. The unmarked theme is that which can also be a person who does something in a clause. For example; "Wayne hit his brother on the head." The theme in this case that we are asking is who hit his brother. We did not set out to make two themes but the nature of the clause makes the secondary theme. One knows that the other theme is that the brother was hit. Both are unmarked themes. A marked theme tells one directly what a clause is about.
The problem arises often in language translation. It is difficult for a translator to change the wording to project the same meaning and response when word order directs the clause in different languages.