How to set Video Background in iMovie
The black background is just because black is the absence of any other image there. You can make a white jpeg photo with Photoshop or Inkscape or whatever program you have, insert it as a frame, and then it's white. Then you can drag title text onto that white image.

You can also drag title text as a new clip, and then it gives you an option of various backgrounds (including white).

For the rest of the video, well, you see video as the background, not any color.

If you want more control of layers and opacity and backgrounds and such, you may want to consider a step up to Final Cut Pro.