One theme, the main theme, is the emptiness caused by lost love in an immense universe. The two ideas of love and universe are tied inseparably together in Neruda’s poem. The poetry "falls" into his soul because of "the immense night, more immense without her." The night wind revolves and sings and the stars shiver, so that on this night he can write the saddest poem. Tonight, he can write the saddest lines about stars and immensity because he feels he has lost her.
His soul, however, is not satisfied to think that "it has lost her." In the distant immensity of stars and night, his soul is not satisfied, and still his eyes and heart search for her. On other starry nights, he used to hold her, and maybe he still loves her, but now, under a night sky, with singing in the distance and starlight falling on the same trees, his soul is not satisfied without her.
There are multiple English translations of "Tonight I Can Write (The Saddest Lines)," including one by American poet and Pulitzer Prize winner W. S. Merwin, who lived in Spain and was a prolific translator:
Tonight I can write the saddest lines.
Write, for example, 'The night is starry and the stars are blue and shiver in the distance.'
The night wind revolves in the sky and sings.
Tonight I can write the saddest lines. (Merwin)
These lines emphasize the role of the immense universe, "more immense without her," in the feeling of loss and emptiness the poet feels: "my soul is not satisfied that it has lost her."