The theme of Keats' sonnet "On the Grasshopper and Cricket" must be derived from textual evidence. Three things point to the theme. The first is the title in which Keats introduces specifically the grasshopper resplendent in summer and the cricket that endures the winter. In the contemplations of the poem, Keats describes his awareness of, his experience of, the grasshopper and then the cricket.
The second thing is the opening line of the sonnet, which is repeated at the 9th volta, or turning, line: "The poetry of earth is never dead." Keats is focusing our attention on the music, the "poetry," of nature; bear in mind that, for Romantic poets, nature contained truth that inspired poetry, truth that poetry sought to imitate or reproduce.
The third thing is the final triplet of the poem (the last three lines). Keats speaks of the "Cricket's song" and of the "drowsiness" of being half asleep and of the similarity of the cricket's song to the grasshopper's ("And seems to one ... / ... / The Grasshopper's [song]"). Keats also speaks of being drowsily transported to summertime and the sound of the grasshopper singing, perhaps from under "some pleasant weed."
When you consider these three things, the theme begins to emerge. Keats speaks of poetry, voice, song; he speaks of poetry never ceasing; he equates the grasshopper with the cricket by imaging the one while hearing the other; and he equates poetry with the songs of the cricket and grasshopper through the word "voice."
One might be tempted to think this sonnet reflects the theme of the "cycle of life," but this theme has no correspondence to poetry, voice, song, which is the music of nature. A more correct analysis of theme would be that Keats is expressing the idea that the trivial, insignificant small things of nature comprise nature's poetry and song: grasshoppers and crickets are very trivial, small and insignificant. Yet it is the song and poetry (remember, song and poetry are equated through "voice") of the grasshopper and cricket that carry on when all else are done.
Keats says that after the birds ceased their singing in the hot summer, the grasshopper could still be heard:
When all the birds are faint with the hot sun,
And hide in cooling trees, a voice will run
From hedge to hedge about the new-mown mead;
Keats also says that "when the frost / Has wrought a silence" in winter, the cricket can still be heard on the hearth, its "song, in warmth increasing ever." In this sonnet, Keats is putting forth a theme that reflects the Romantic period belief in the inspiring truth of nature by showing that even when all nature seems overcome by heat or cold, there is still the poetry and song of nature that gives truth and inspires:
The poetry of earth is ceasing never; / ... /
The Cricket's song, in warmth increasing ever,
And seems to one in drowsiness half lost,
The Grasshopper's among some grassy hills.