One Hundred Years of Solitude is full of many different themes. Because the book is so dense and covers such a large span of time, it can be difficult to focus in on one theme.
However, one that does stand out is the theme of family. This novel follows family through many different generations and time periods. The family is important, but Marquez also shows some of the more negative aspects of family. There are arguments, differences of opinion, and so on but there is also love, sex, birth, and growth.
When looking at the theme of family in One Hundred Years of Solitude you could take it either way, and you could write about the ways in which family harms itself or about the ways in which family outlasts struggle and war. Alternately, you could focus on the strange ways in which family stays together despite the odds.