Due to limited access to both poems, below are some ideas to help get you started.
Two dominant themes in Derek Walcott's poem "Ruins of a Great House" are certainly death and decay. Death is first alluded to in the lines of Sir Thomas Browne quoted before Walcott begins his own poem, particularly in the clauses, "[I]it cannot be long before we lie down in darknes, and have our light in ashes"; the lines are quoted from Sir Thomas Browne's book fully titled Hydriotaphia, Urn Burial, or a Brief Discourse of the Sepulchral Urns lately found in Norfolk. Death and decay are referred to again in the first two lines of Walcott's poem in reference to the "disjecta members," which means in Latin scattered members, of "this Great House" and the "moth-like girls." We even see death and decay being referred to in several images, such as "dead leaves" and "rotting lime."
But the poem itself is about much more than just death and decay. There are several allusions in the poem that make clear references to colonial slavery, and the speaker ends by feeling a unity between what was once the slave master and all of humanity, including the poem's speaker.
The first reference to slavery is evident in the reference to the "Great House," which is clearly a manor house, and slaves worked its lime orchards. A more obvious reference to slavery is found in the first two lines of the final stanza: "Ablaze with rage I thought, / Some slave is rotting in this manorial lake." In the second-to-last stanza, more subtle references to slavery can be seen in the allusions to "Hawkins, Walter Raleigh, [and] Drake," which the poet calls "[a]ncestral murderers and poets." The name Hawkins refers to David Hawkins, a philosopher who was also the historian of the Manhattan Project, which developed the nuclear bomb, enslaving all of the world to its deadly powers. The name Walter Raleigh refers to Sir Walter Raleigh who during Queen Elizabeth's reign first explored Virginia, exploiting Native Americans. Finally, the name Drake refers to Sir Francis Drake, who was also an explorer and slaver during Elizabethan times.
It's also important to note that the poem ends with an allusion to John Donne's poem "No Man is An Island," which argues all of mankind's sorrows are connected due to mankind's unity. Hence, all in all, the poem seems to be arguing that, despite the fact that slaves and slaveholders have existed, all mankind is connected in death. Hence, some of the poem's themes include death, decay, slavery, and unity of man.