The Giver has several themes. The most important themes of the story are:
Coming of Age Twelve is the most important time in these characters’ lives. After that, age is not considered important. This is the age to leave childhood behind, and at this time, your life’s calling is decided.
Sex is a purely “mechanical affair” in which girls are chosen at age 12 to become birthmothers and spend 3 years at the Birthing Center having 3 babies that they do not raise, but who are given to volunteer families. Sexual urges are suppressed in all others.
Death is referred to as Release from the community and going Elsewhere.
There is not one main theme, but the major themes are:
Coming of Age
The Individual vs. Society
If you had to pick one, you could say "Coming of Age", because this is central to the main character, Jonas' journey through the book.