The most prevalent theme in "Africa" is that of enslavement. Throughout the poem the author provides images that support this theme:
"two Niles her tears" expresses Africa's great sorrow. (I believe Africa is personified as a woman: the "mother land.") "Brigands" offers the image of slave traders who "took her young daughters / sold her strong sons / churched her with Jesus / bled her with guns."
The second theme I see is of emancipation:
"Now she is rising..." and "now she is striding." These images give a sense of freedom—unshackled movement.
The final theme seems to be remembrance...a respect for the beauty and history of Africa. The land is described as...
"sugarcane sweet / deserts her hair / golden her feet / mountains her breasts..."
We can imagine how the land looked before the slavetraders arrived.
Remembrance is echoed again in:
"remember her pain / remember the losses / her screams loud and vain / remember her rishes / her history slain..."