Kraatz

Hi all: I'm new to site building and am confused by something. I've decided on using WordPress and Gutenberg for the block builder. My first site is going to have a pretty simple layout. I don't want any of the razzmatazz themes out there. And this brings me to the question. It seems to me that I can build the site I want without having to get involved with attaching a theme program to Gutenberg. That I can do what I want with Gutenberg alone. Now, I've read where a builder has to have a theme, but I don't see why, unless wanting a specific theme. Am I wrong about this? Thanks for replies