How to set title background transparent in the iMovie 10.0.6 for the application preview

Question

For the iOS8 application, the AppStore have to upload the preview. With the Yosemite, I use the newset iMovie(10.0.6) to make the application preview. I just want to add some title or text over the video, but i find something dispointed. There is no way to set title background color transparent, that is impossible.! ![]