The phrase "a theme would be" is correct and usable in written English.
It can be used when introducing or suggesting a possible theme for discussion, analysis, or exploration in a piece of writing.
Example: "In this essay, a theme would be the impact of technology on human relationships."
Alternatives: "one possible theme is" or "a potential theme could be".
A restaurant with a historic theme would be opened in the once-famous No-Name hat factory, which was owned by Stetson Brothers.
Mr. Monter said that a nautical theme would be reflected in a gatehouse flanked by three ponds and designed to emulate a lighthouse -- with a working light.
A better theme would be the suffering of innocents.
Another show with a transportation theme would be "The Local".
Febre said that since he began composing for Smallville there was a shift to "thematic development" in the score, paralleling the characters' growth: "As Clark grew emotionally and intellectually more complex, I found a need to comment musically on his growth, and as he drew closer to his Superman persona, it became obvious that a 'Superman' theme would be required".
He was brought up in Fiji and spent his twenties working as a sports agent in Hong Kong, so a dinner with a pan-Asian theme would be appropriate.
A more specific theme would be "cats knocking things off surfaces," and all your Vines would have to illustrate cats knocking various objects off of various surfaces.
Although Zombie has added more history to the Michael Myers character, hence creating more original content for the film, he chose to keep the character's trademark mask and Carpenter's theme song intact for his version (despite an apparent misinterpretation in an interview suggesting the theme would be ditched).
A variation on the theme would be to give parliaments the ability to force the commission to review existing legislation, not just new rules.
