To find the themes of "Frederiksted, Dusk" by Derek Walcott, let's start with trying to sort out the meaning of this complicated poem. It is a kind of transcendental poem that addresses the transcendence of life over death by a poetical examination of objectified life experiences (e.g., staring at sunset; tanning on a rock) and by philosophically reflecting on that which has been objectified, thus transcending (rising above) objectifications.
The poem starts out like a narrative with perhaps the commonest of all sorts of people as the protagonists, old men "set down ... outside the almshouse," or the poor house. The narrative-like current of the poem goes on to compare the old men's "level stare of light" to a "girl tanning on a rock" who "fills with light," connecting them again further down the poem with references back to the light-filled, youthful girl: "would shine in them"; "substantial light and insubstantial stone."
The transcendent process begins right at the start of the poem in the opening metaphor and simile Walcott employs: sunset is a picture show; old men are like empty bottles set down in the morning (like empty milk bottles set out to be picked up). Walcott then launches a series of completely impossible figures of speech, a trope called catachresis: "the rising evening brim their eyes"; "level stare of light"; "more than mortality brightened air"; a girl who "fills with light."
The narrative fades out after the girl full of light on the rock and the focus changes to philosophical musings on "Whatever it is / that leaves bright flesh like sand," thus transcending the objectified narrative. The main topic is death ("leaves ... flesh ... and turns it chill"), but it is death with an incongruous twist examining the complex realities between the "simplicities" of "life and death."
This "Whatever it is" is a "state," a "collective will," that shines--like the light that fills the girl--in the dying old men. The "state" waited ("a state ... and it waited") like the girl also waited ("it waited too") in the incongruity of substantial light and insubstantial stone. The message derived from the poem is that human inner being, inner light, transcends over both the substantially real misfortune of being set out like an empty bottle in front of almshouse and the equally substantially real rock to lay and tan on.
The themes now emerge a little more clearly. The major theme is as stated before, that being the transcendence of life over death. Another is the transcendence of humanity's inner light over objectified reality. Another is the shared resource and essence of light: the sunset and the old men shared a stare because they share essential light, one source of the shared light being the Sun, the other source the inner being of humanity.