You can create a style repository from any Help+Manual project. You just need to select Save As... in the File menu and then select Save as Repository as the project type to save. You need to save to an empty folder, just as you do for a normal uncompressed HMXP project. After saving you need to "register" the style repository with the Repository Manager, which makes it available for use in your projects.
The objective should be to edit your repository styles in the style repository, not in the projects linked to the style repository. This ensures consistency across all projects using the style repository.
The purpose of creating a style repository is primarily to have a standard set of paragraph and text styles that you can share between multiple projects. The style repository links and imports these styles to the projects where it is used, and you can update the styles for all the projects by editing the styles in the style repository.