There is no feature that lets one change the basic black/charcoal color of the iMovie 10 app. project screen. Most people prefer the dark background because it makes the clips stand out more.
You can drag a white background to the timeline and overlay a clip onto it with the Picture in Picture feature, and then make the overlay slightly smaller than the dimensions of the preview screen. That would give you a white boarder around the clip. Not much advantage to doing that unless you like the effect.
EDIT: You can also go to Systems Preferences/Display and increase the brightness of the Mac screen, if that would make it clearer for you to see.
-- Rich