What is the theme in The Thirty-Nine Steps by John Buchan?

The art of the disguise is the main theme of this story. From his initial masquerade as a milkman on, the protagonist Richard Hannay takes on a dizzying amount of disguises during his adventure. Sometimes Hannay's transformation seems to happen merely on the surface: he changes clothes with the milkman, and this uniform alone is sufficient to fool anyone on Hannay's tail and allow him to escape to the train station. Similarly, the simple adoption of a thick Scottish brogue on the train ingratiates him to the lower-class Scots on the train, allowing him to blend in. However, Hannay's disguises can be shown to progress, to where mere appearance is not sufficient enough.