When the camera is selected it shows bars at the top and bottom of the viewport to show the area in between that will be rendered according to the camera Resolution. Is there a way to make them darker (less transparent) to make it easier to see them?

Hit "d" when you're mouse is over the viewport to open the Display Options.
Then under the "Camera" tab you can set the View Mask Opacity.

Cheers

I found in v18.5, the setting is in Display Options (D) in View tab -- the View Mask Opacity control.