In "Ode to My Socks," what is the collective effect this poem has? What is the theme?
The poem celebrates the appreciation of everyday things; namely those which have practical functions. The narrator is so appreciative that he acknowledges who made them (a subtle intimation in reference to his more political poems) and he makes analogies and metaphors that are almost like a child with a new toy or a new pair of shoes.
Neruda wanted to touch upon things that are important and significant to common people, so it is part of his overall political (communist) commitment to, not just common people, but the utility of what they make with their own hands. He describes the socks as more than ordinary, magnificent really. He praises them so much to underscore the appreciation of everyday things (stop and smell the roses) but also in preparation of the turn in the poem where he notes they are to be used; not just praised as if they were valuable collector's items or jewels to be encased in glass.
They are doubly beautiful and good because there are two of them. Easy enough. But they are doubly good and beautiful because their beauty lies in their utility (goodness); they are good because they help you stay warm. They have a purpose. In this context, there is more meaningful beauty in the utility of socks than there is meaningful beauty in the utility of a golden statue.