It's easy to change the borders - just change the "background" color on the slide. (The white part in the middle is actually an opaque "text box" superimposed over the background).
Just right-click anywhere on your slide, choose "background" and change the color. Or, go to the Format Menu, then select "Background" and change the "background fill" color. I'll be using different colors for different units, because I think that makes more sense for my content-specific vocabulary. (As opposed to the way elementary uses one color for math, and one for language arts).
Enjoy,
Jen