I checked the link provided by FumbleFingers in a comment above—and found not 48 but 3 verifiable Google Books matches for "grand scheme of themes"—one from 2014 and two from 2016. Let's look at how each one uses the expression.
First, Antonio Jacobs, The Yellow Flick Road: How All Movies Are Based on The Wizard of Oz (Lulu.com, 2014), has this:
Reality versus fantasy: that is what it boils down to. In the grand scheme of themes, there are two worlds: the dry, mundane flat landscape of a barren monochromatic Kansas, and the wild, diverse and exciting Technicolor world of Oz. Both worlds are fraught with danger: In Kansas, on the farm, Dorothy falls into a pigpen, needing a quick rescue. If movies like Hannibal are any indication, falling into a pigpen is sudden death.
Theoretically this self-published author might be knowingly engaging in catachresis, in accordance with Lambie's answer. Alternatively he might be making a simple mistake and not realizing it. The latter hypothesis is strengthened by instances of other idiomatic blunders and odd word choices ("Call it an obsession, but now I can watch the worse movie ever made and get some truth out of it"; "the leader, the techie or computer nerd that more than often saves the day with some sort of gizmo or hair-brained stunt"; "Lewis Carroll is credited for the invention of la femme fatale in Alice from the classic Through the Looking Glass, but it is Baum who perfects the archetype"; "The Cowardly Lion is our resident psychotic").
From Dennis Cooley, The Home Place: Essays on Robert Kroetsch's Poetry (University of Alberta Press, 2016):
Kroetsch finds beauty and plenitude and vitality in a world that others find scant. Though in the grand scheme of themes Kroetsch's realities may seem small and marginalized, they are enormously alive—vulgar, raw, profuse, obscene—and they put a charge into the poem. Dirty words in the sacred books. In story, Kroetsch has argued time and time again, we become ourselves and we do so disruptively: ...
This book lists a copy editor/proofreader, so the author may have prevailed with the catachresis argument, although I wouldn't put the odds at better than 50-50 that either person had a clear sense of the idiom "grand scheme of things" and went with "grand scheme of themes" for effect.
And from Woody Leonhard, Windows 10 All-in-One for Dummies (John Wiley & Sons2016):
Rollback: If you figure you just got hit with a bad update to Win10, it's now easy to roll back to the previous version, or build, of Win10. The most recent build, which may include bad device drivers or other tweaks that make things go bump in the night, return to their previous incarnation. That's a hiccup, in the grand scheme of themes.
Unlike the exegesis performed on Kroetsch's poetry, Leonhard's narrative isn't even remotely about themes. He simply used the wrong word in his idiom, just as he wrote in the previous sentence (once you remove the lengthy independent clause) "The most recent build return to their previous incarnation." There is nothing in the quoted paragraph that a copy editor couldn't fix in short order. But I get the impression that Leonhard didn't have any backup.
A search for "grand scheme of themes" on the wild Internet at large, yields several dozen matches, almost all of them from people who do not seem to be knowingly playing on the standard idiom for heightened effect.
Most instances of "grand scheme of themes," I suspect, are like most instances of "for all intensive purposes," "nip it in the butt," "doggy-dog world," and "deep-seeded belief": they sound enough like the normal idiom that some people think that the variant is what everyone else is saying—or should be saying. The simple term for that phenomenon, as FumbleFingers observes, is mistake.