By default, all movie projects have the Simple theme applied. You can change your movie project’s theme at any time. Each iMovie theme includes coordinated titles, transitions, and music that make your project look professional.
With a movie project open, tap the Project Settings button 
.
Tap a theme to select it.
Tap Done.
When you change the theme, visual theme elements (such as titles and transitions) and theme music in the project are changed to match the new theme.
When you turn on theme music in project settings, the music is automatically faded out at the end of your project. As you edit your project, the point at which the theme music fades out is adjusted to fit the project duration.
With the project open, tap the Project Settings button .
Tap to turn Theme Music on or off, then tap Done.
You can also add theme music to your project separately, in the same way you add a soundtrack. See Add soundtracks and other background music.