“Remember” is an elegiac poem, focusing on the themes of death, remembrance, relinquishment, and forgiveness. The speaker is Rossetti pondering her impending death and releasing her lover from the responsibility of enshrining her in his memory because she fears it will cause him pain. She tells him that if he chooses to forget her, he should not feel guilty.
In addition to the themes of elegy and imperfect love mentioned in the previous answers from other educators here, there is possibly a theme of existentialism at work in this poem. An existentialist view generally accepts that life is without meaning or purpose, that things happen for unexplained but banal (rather than mysterious or mystical) reasons.
Using this interpretation, once can see that there is very little in the sonnet that is not based upon the real and final condition of death. The word "darkness" implies a state of nothingness and loss of consciousness after death. The word "corruption" is a reference to the earthly decay of the body after death, and therefore the idea of what occurs after death is focused on those left behind (the poem's intended audience), and not the fate/future of the poet.
And yet, being a thinker and artist, the poet's words here are bound up in ideas of ego and remembrance, perhaps partly because of her body of work and its legacy that she hopes will live on. The sonnet's language makes no explicit reference to this, only a somewhat vague description of "a vestige of the thoughts that once I had."
The theme of imperfect love is also present in Rossetti's "Remember." A very religious poet, Rossetti here seems to realize the imperfection of human love against that of the divine. Thus, her relationship with her lover may not be as sincere as it first appears in the first eight lines of this Petrarchan sonnet. For, in the last six lines, Rossetti's attitude seems to change to one of nonchalance:
Yet if you should forget me for a while/And afterwards remember, do not grieve:
Rossetti tells her lover that the imperfection of their love allows for forgetting:
For if the darkness and corruption leave/A vestige of the thoughts.../Better by far you shoul forget and smile/Than that you should remember and be sad.
Also, because the opening lines do not clarify why the speaker is leaving--is it separation or death?--the ending lines seem all the more nonchalant if the reader considers that the speaker may simply be leaving.