"a principal theme in" is a correct and usable phrase in written English.
It is typically used to describe a significant or central idea or topic in a given context. Here is an example sentence: "A principal theme in the novel was the struggle for power between the two main characters."
That's a principal theme in the most surprising movie hit of the year so far, "Get Out," whose box office haul in America crossed the $100 million mark last weekend.
Lately, resentment of this sort has again become a principal theme in international economic relations (the row over the new head of the World Trade Organisation, where America is also accused of throwing its weight around, is another example).
His first-hand knowledge of the countryside's "beautiful green solitude" provided a principal theme, one illuminated in the peasant patois of 19th-century Northamptonshire, where a snail was a "pooty".
Fugue, in music, a compositional procedure characterized by the systematic imitation of a principal theme (called the subject) in simultaneously sounding melodic lines (counterpoint).
Conversely, in a biographical sketch of Powell, Michael Barber believes that Widmerpool's demise accords with a principal theme of the novels, expressed in Casanova's Chinese Restaurant (fifth book in the sequence), that "in the end most things in life perhaps all things turn out to be appropriate".
The inquiry is an issue in the Philippine elections, where corruption is a principal theme of the opposition campaign against Mr. Marcos.
In Eritrea, Rossi discovered another phenomenon that would become a principal theme of his postwar cosmic ray research: extensive cosmic ray air showers.
A principal theme of medieval pornography was the sexual depravity (and hypocrisy) of monks and other clerics.
Her transformation "from brash warrior to a spiritual being", according to DiMartino, is a principal theme of the series.
