Hi Johnny,
You need to select the "Range" button in the depth display section, before the depth-range min and max fields becomes enabled. Then you should be able to enter min and max depth values, and the depth colours will update as you change these values.
I think you may have the opposite problem with minor contours. You have selected "generate in range" but your max and min depths for minors are both zero, so no minor contours are being generated. Just click the "Range" button once in that section to turn that feature off and create minor contours over the full depth range (or, alternatively, enter an appropriate depth range in the max and min fields).
Matt