One of the main themes is probably the exploration of human - animal relationships. It shows how humans treat animals and what that says about their own humanity.
A third theme might be the idea of the outsider. The dog, being different from humans, is an outsider in the human world. This can be used to explore how outsiders are treated, and how they struggle to fit in or find their place in a world that is not made for them.
Perhaps the theme of sacrifice is present in 'the heart novel'. Characters may have to give up something they cherish, like a dream or a relationship, for a greater good. And social inequality might be another theme, showing how different characters are affected by the social structure in the story.
One of the main themes is imperialism. The novel shows the dark side of European imperialism in Africa, how it exploited the land and the people. Another theme is the loss of innocence. As the protagonist travels deeper into the Congo, he witnesses so much horror that his own sense of self and innocence is shattered.
One of the main themes could be courage. Given the title 'warriors heart', it might be about characters showing great courage in difficult situations.
One possible main theme could be love. In many novels, love often plays a central role, whether it's romantic love between characters or love within a family or community. Another theme might be the unique Sardinian culture. The setting of Sardinia can bring elements like local traditions, values, and ways of life into the story.
Love could be one of the main themes. Imperfect hearts often struggle with love, whether it's finding it, keeping it, or dealing with the pain it can bring.
One main theme could be cultural identity. It might explore how characters in the novel are shaped by their cultural background. Another could be the concept of change, perhaps in relation to the society depicted in the story.
Courage is likely a main theme. The title itself suggests that the characters have fearless hearts, so they must be showing courage in different situations.
The main themes might include the exploration of human emotions. Since the heart is seen as the seat of emotions, the novel could delve deep into how different characters experience and cope with emotions like joy, sorrow, anger, etc. Also, it may touch on the theme of relationships. How the concept of the heart affects the way characters interact with one another in various relationships, be it friendship, enmity or love.
Another theme is social class. We can see how different classes are treated and how they interact. The poor often face more hardships and less leniency in the eyes of the law compared to the wealthy.
Love and sacrifice could be main themes. In many novels, a red heart can symbolize deep love or a passionate spirit, which might lead to characters making sacrifices for their beliefs or loved ones.