The theme of the poem is

Although the poem uses imagery of autumn, twilight, and dying fire to depict aging and approaching death, the concluding couplet reveals the true theme: 'This thou perceiv'st, which makes thy love more strong, / To love that well, which thou must leave ere long.' The poet is telling his beloved that recognizing his approaching death should make their love stronger. The central theme is LOVE — specifically, love made more intense by awareness of mortality.