Does the theme in the book Siddhartha reinforce or oppose popular nations of life?
I'm not sure what "popular notions of life" are, but Hesse worked hard to create a timeless, yet modern duality to his novella. Like the river itself, the novel begins with man's eternal spiritual quest and it meanders along a modern existential course until it finds a balance between the two.
The most modern "popular notion" of philosophical/religious thought has been of the existential variety, stressing individual freedom over traditional "group follow." Walter Kaufmann claims existentialism is by nature a "revolt" against traditional philosophy, not "reducible to any set of tenets," instead relying on a "timeless sensibility" only recently evolved into a "sustained protest."
Siddhartha, in his spiritual quest, foresakes the Buddha and his followers because Siddhartha wants to discover for himself, firsthand, a new spiritual path devoid of a past, "to leave all doctrines and all teachers and to reach my goal alone--or die" (34). To follow another doctrine is to "deceive myself that I was at peace and had attained salvation, while in truth the Self would continue to live and grow."
This "anxiety of freedom" from which Siddhartha suffers seems to allow for a kind of spiritual free agency. One may indulge the self, while borrowing the most applicable parts from the teachings of the major religions, to create a secular, quasi-religious harmony.