You can use any of your workspace themes in your project, by selecting the theme you want in the “Theme” tab of the project editor.
By default you will have the two ‘default’ Shorthand themes, or if you have a custom branded theme it will appear there.
This theme will then be applied to every story in that project.
Please note: All stories in a project must have the same theme. If you add a story to a project that has a different theme, it will be updated to the project theme.