The phrase "a principal theme" is correct and usable in written English.
It can be used when discussing the main idea or central topic of a piece of writing, artwork, or discussion.
Example: "The novel explores a principal theme of love and sacrifice throughout its narrative."
Alternatives: "a main theme" or "a central theme".
Even as his plays changed and grew, that continued as a principal theme.
Loyalty is a principal theme as Löw searches for the solutions against Sweden.
A principal theme of medieval pornography was the sexual depravity (and hypocrisy) of monks and other clerics.
Boullée's emphasis on the psychology of the viewer is a principal theme of his Architecture, essai sur l'art, not published until the 20th century.
The "appliance of science" will be a principal theme of the strategy for manufacturing that the DTI is now drawing up.
Fugue, in music, a compositional procedure characterized by the systematic imitation of a principal theme (called the subject) in simultaneously sounding melodic lines (counterpoint).
Mr. Bush's principal theme once again was that the vice president wanted to exploit issues for political gain rather than solve problems.
And Mr. Anderson's "Physical Cities" was the second tour de force; coming right after the Bowie tune, it managed to raise the bar, moving episodically between stark syncopations and a flowing principal theme.
Where Stravinsky's inspiration seems to fail him — at first encounter, at least — is with the entrance of a nobly sorrowful principal theme, which is first heard in the solo horn and then migrates to the strings and elsewhere in the orchestra.
The design and support of the central dome or cupola, together with the number and disposition of the subsidiary cupolas, remained for a long time the principal theme of Russian architecture.
His early political works included The Chains of Slavery (1774), an attack on despotism addressed to British voters, in which he first expounded the notion of an "aristocratic," or "court," plot; it would become the principal theme of a number of his articles.
