Q: How do I add a theme to imovie after I have started the project?

unfortantly you cant but copy the movie and start a new theme then pasta it

You can select the "Settings" button. This is found in your window under the preview pane. (If you see the area where the video plays back, it is directly under the "Play full screen" arrows.) Once in the Project Settings, you can change the theme as often as you like, but it may not impact the transitions you already have in place, so those will have to be updated manually.