What are the main themes in "A Man of the People" by Chinua Achebe?

The main themes are as follows:  the struggle for power, the influence of greed, and the prevailing aura of corruption.

The struggle for power can be seen through both politics and through generations.  In regards to politics, Odili has avoided the influence of the powerful Chief Nanga.  However, he is slowly enticed to Nanga's side through parties, vacations, and other bonuses.  In regards to the generational struggle for power, one needs to look no further than to Odili and his own father.  The father lives in poverty due to the influence and actions of his many wives.  Odili wants more for himself than this poverty, but the irony is, it seems that the way out of poverty is the connection with Nanga that Odili has always avoided.  It isn't long before he finds out that there are strings attached to every loyalty.

In regards to the theme of greed, there is no doubt that Odili wants more than the poverty of his father.  Even though Odili is disgusted by Chief Nanga at the beginning, he is slowly seduced by the wealth involved.  Being invited to ostentatious receptions and parties and mansions truly call to Odili.  Odili's greed is ignited.  After independence from white rule, the country divided its natives into two groups:  the majority were poor, but there was a tiny minority who would serve the whites and, therefore, became rich.  Bribes are prevalent.  People (even those with government sanctioned jobs) are paid to turn their back on injustice. What beckoned to this select few who have more than enough?  Greed.

Finally, connecting the two themes above is the theme of corruption.  Note this pithy statement:

[Nanga is] bloated by the flatulence of ill-gotten wealth, living in a big mansion built with public money, riding in a Cadillac, and watched over by a one-eyed, hired thug.

There is corruption in government (due to bribes and injustice and racism) and there is corruption in financial matters.  Look at the corruption of Chief Nanga:  he continually uses deceitful practices and distribution of wealth to gain followers.  Lavish parties and visits to mansions are the norm for those that give in.  Jobs teaching the poor of the "bush" are what remains if you don't give in.  Closely connected to this is the corruption in finances.  Only those connected with the white ruling class (before and after independence) have money to throw around.  Everyone else is either poverty stricken or just scraping by.