Another theme could be hope. The dreamer in the story must have had hope to pursue their dreams against all odds. It might also involve the theme of self - discovery as the process of chasing the dream could lead the person to learn more about themselves. For example, they might discover their hidden strengths or weaknesses while striving towards their goal.
I think inspiration could be a key theme. A true story about a dreamer is often meant to inspire others. It could also touch on the theme of transformation. The dreamer might have transformed themselves or their surroundings through their pursuit of the dream. Maybe they started from a humble beginning and through their hard work and dedication, they were able to change their own life and perhaps even the lives of those around them.
Possible main themes could be perseverance. Since it's a true story about a dreamer, the person likely had to keep going in the face of difficulties.