Work with themes in iMovie on iPhone
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.
Add a theme
With a movie project open, tap the Project Settings button .
Tap a theme to select it.
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.
Turn theme music on or off
When you turn on theme music in project settings, the music dynamically adjusts to fit the length of your project.
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 music and soundtracks in iMovie on iPhone.