No, there is no tool to make themes. Themes are done with standard web techiques like HTML, CSS files with a little PHP or ocassionally a little JavaScript depending on what the theme does or you want to do. Please read the theming tutorial to learn how Zenphoto themes work.
Edit: Do you mean the themes editor on the themes tab on the backend maybe? That is a simple source code editor and a real editor would be much more convenient I would think.