The Conflict between Necessity and Moral Norms
This is the novel's main theme and this can be seen on throughout the narrative. It is this dynamic between the two seemingly diametrically opposed concepts that moves the characters and drives the story forward. All the characters at one point or another struggle between the striving to maintain a degree of human morality and/or deciding to cast aside established moral norms for the sake of survival. Bill works to rescue the blind, assisting them when he can because he still adheres to his moral code. This code both defines him and dictates his actions. Beadley is on the polar opposite of this moral spectrum seeing survival as the highest objective that he must strive to achieve. Looking at situations from that angle he therefore sees the blind as a waste of resources and this dictates his responses towards other survivors he comes across.
Hope
Despite the bleakness of the novel there is a definite undercurrent of optimism that is present in that the protagonist hopes against the situation that things will improve. It is this hope for a better tomorrow that keeps him from giving up on his search for Josella. It is this hope that they will eventually find a means to eradicate the triffids and reclaim the earth that pushes him to keep on surviving.
The Fragility of "Human Domination"
The author uses both the triffids as well as the plague of sudden, inexplicable blindness to illustrate how tenuous humanity's domination of the world truly is. In the novel a simple change in a "normal condition," which is in this case humanity's reliance on sight, when abruptly removed on a large enough scale would wreak havoc on our society, quite possibly enough to destroy it altogether or in the very least alter it in ways that would render it unrecognizable from it's current form.
