What is the theme in The Day of the Triffids by John Wyndham?

The Conflict between Necessity and Morality is the narrative's central theme, playing out on several levels. First, it motivates the action, with events in the plot and the steadily increasing sense of narrative tension throughout the book all being defined by the tension between the striving to maintain a degree of human morality and the necessity to cast aside that morality in order to survive. Further to that point, the individual struggles of the characters (which here, as in all good narratives, define the movement of the plot) are themselves defined by their individualized wrestling with the question of survival vs. principles.