Two very significant themes are introduced in the beginning of the play. They are (1) post-colonial unrest and rebellion and (2) wealth and poverty. These two are undergirt by a third theme, and (3) that is tradition and traditional ways.

Post-colonial unrest and rebellion is dramatically introduced through the hymn sung in Kiguunda and Wamgeci's yard. It is sung by a group raising a haraambe, or a public fund, for establishing a new sect and new church, the focus of which will be on crushing the "Satan of theft," the "Satan of robbery," the one who "oppresses the whole nation." The leader and followers protest against the poverty that has throttled their country and lives since the British withdrew.

The Leader:
We belong to the sect of the poor
Those without land,
Those without plots,
Those without clothes. We want to put up our own church.

The theme of wealth and poverty is represented by Kiguunda's title-deed to "one and a half acres" of his own land and by the drunken life of Kamande wa Munyui who, after losing two jobs, falls into a state of drunken poverty. The post-colonial rebellion sect also represents the theme of wealth and poverty, as is further illustrated by the plea of the leader who asks for whatever money Kiguunda and Wamgeci have set aside. The theme of the title is ironically introduced along with that of wealth and poverty when drunken Kamande wa Munyui sings the associated song at Kiguunda and Wamgeci's door. This is ironic because wealth is needed to marry when you want.

Drunk:
I shall marry when I want,
Since all padres are still alive.
I shall get married when I want,
Since all nuns are still alive.

The undergirding theme of tradition and the traditional ways of the Kenyan people is introduced when Kiguunda's mood lifts and he wants to remind Wamgeci of how he won her love by dancing the Mucung 'wa sword dance. In his sudden lighthearted feeling, he sings and dances for Wamgeci and Gathoni. This song and dance ties all three, both generations, to the traditional ways of old that were freely lived before the British imposed a state of emergency that began in 1952 and ended ten years later in 1962.

The author Thiong'o is a distinguished professor of comparative literature at the University of California, Irvine. He is also a known theorist and author. When he wrote this particular play, it caused quite a bit of controversy because it pointed out the poor living conditions of the common people in Kenya. 

The play weaves his interpretation of the living conditions and culture clashes of post-independence Kenya, when land and wealth were supposed to be returned to the indigenous people, into a somewhat comedic story about a family struggling to keep their home and land. Although told as a story about this family, the author focuses on the political stories throughout the play so that the main themes stem from this aspect.

The main themes are betrayal and conflict based on the conflict between commoners and those in charge. The indigenous commoners were betrayed by those in charge and seemed to have less after British rule ended than they did before when the opposite was supposed to be the case. Conflict arose as some of the Africans joined the "Homeguard", siding with the European interests and took action against fellow Africans. 

"I Will Marry when I Want" is a play about post-imperialism in Africa. It deals primarily with the themes of greed, neo-colonialism, and the cultural and religious acrimony produced by the conflicting traditional and imperialist factions in modern-day Kenya. The story centers around Kiguunda, a farm laborer, and his wife, Wangeci. When Christians tell Kiguunda and Wangeci that they should sanctify theirmarriage in a church, they reluctantly agree, preferring instead to maintain their traditional values and customs. However, the realize they cannot afford such a ceremony without selling their half-acre of land to the white imperialists. Essentially, the play examines the motives and greed of the imperialists and the Kenyans who have been corrupted by this new way of life.