I'd suggest that to find a theme, you could temporarily step away from the specifics of your main character (the one born in 1855--your great-grandfather, perhaps?) and ask what most matters to you about his life.
Did he have some outstandingly brave character traits? OR did he endure many hardships without succumbing to despair? OR was he the kind of person who always had a joke and a good word for everyone? OR was he a character who often slipped into trouble but usually managed to escape? In other words, what quality, in general, do you think was the center of his character? Focus. Choose a main one.
Here's another exercise: What three or four adjectives would you use to describe him? For example, people might describe the familiar figure "Satan" as deceitful, vengeful, and cruel. (Not suggesting these for your relative!)
You're looking for a theme that is like a strong and general sentence to describe what this character is like, and why he matters to you, something like...
My g-grandfather was the most self-effacing man I have ever known, taking credit for none of his accomplishments.
My g-grandfather struggled all his life with illness and death, so much so that he came to question his own faith.
My g-grandfather was an easygoing and carefree person who was able to live in the moment, despite all the hazards of fate that he encountered.
See what I mean? For a theme, create this kind of a statement. There are hundreds+ of possible statements you could come up with. This kind of statement makes a strong, "opinionated," somewhat edgy claim about your main character. Strong enough to interest the reader. This kind of statement makes your character matter, makes him visible, right out of the gate. Create your favorite statement that you think can embrace all/most of the details and facts you have.
Ask me some more questions! Let me know whether this is helping. I'm here to help.