What does "a grand scheme of themes" mean?

Here, from "The all-conquering Wikipedia?" in the Times Literary Supplement, is the sentence, with context provided by the surrounding paragraph:

Equally seriously, a listicle with entries of near-uniform length (five or six pages) has, by definition, no sense of proportion. I am delighted that Lynch has found space for Wisden, Liddell and Scott’s Greek–English Lexicon, and the CRC Handbook of Chemistry and Physics, estimable works all (or at least the first two). My concern is that by granting them the same amount of space – half a chapter each – as Samuel Johnson or the Oxford English Dictionary, Lynch ends up engaging in a kind of historical reverse discrimination. In the grand scheme of themes, as the most sectarian Classicist would acknowledge, Liddell and Scott’s Lexicon is just nowhere near as interesting or important as the OED. To take an even more extreme example, is it not perhaps a shade melancholy to find Edmond Hoyle’s Short Treatise on the Game of Whist receiving more attention (six pages) than d’Alembert and Diderot’s Encyclopédie (four-and-a-half)?

I found "the scheme of things" in the dictionary, but not "the grand scheme of themes."

I understand that the former is frequently used. But I wonder how "the grand scheme of themes" is related to "the grand scheme of things"? What exactly does "the grand scheme of themes" mean?

Solution 1:

The usual idiomatic expression that has been cleverly changed is: (in) the grand scheme of things.

Here, the author has substituted themes for things.

This is a typical literary device. It could be considered a catachresis: The inexact use of a similar word in place of the proper one to create an unlikely metaphor. (Wikipedia definition).

It is probably also some other fancy Greek word which I have forgotten or overlooked.

The grand scheme of themes would be the major issues for Classicists (scholars of Greek and Latin).