Another theme that is developed in The Giver is security versus freedom. The people in the community are safe and secure, with climate control, no diseases, no natural enemies, and no negative emotions such as hate or envy. As long as everyone follows the rules, everyone will be safe from the evils of the outside world, "Elsewhere." As the plot develops, there are countless examples of this, such as being required to report all of one's dreams and feelings, being required to follow the education and profession laid out for each person by the community Elders, and being required to marry the mate chosen by the Elders. But this safety and security comes at a terrible cost, the cost of freedom. No one is permitted to make any choices for him or herself. To have a child or not have a child is not the choice of the individual. When one dies is not permitted to be one's choice. The Elders decide. There are not even choices for clothing or recreation. This is all decided upon by the community, not the individual. As the story goes on and Jonas begins to understand what choices he has been prevented from making, he comes to understand the cost of being safe, and he ultimately decides to take his chances with freedom.
Lois Lowry's The Giver has more than one theme. I'm not sure which theme you are asking about. I'll take a stab at it though with one of the more common and prevalent themes.
The theme of memory is important to the story. Memories can be good or bad. The concept behind The Giver, though, is that all bad and painful memories have been eliminated from society. People no longer remember past events that were hurtful. The only person that is capable of remembering is the Receiver. The elders recognize that there might be a time when those past memories are useful for advice, but not for feeling. This emotionally lopsided society is seen as a good thing. I'm not going to lie, it even sounds kind of nice to me at times.
The theme changes, though, as Jonas learns that he can better understand pure happiness when he is able to compare it to pain and suffering. The theme of memory shifts to showing the reader (and Jonas) that all memories are important. Both good memories and bad memories. That concept becomes so important to Jonas and the Giver that they devise a plan to release all of the memories to people at once.