bounding box use 15

I may be wrong but I don't think you can customize the Move Tool's bounding box lines.  However, you can customize the working space area, i.e., you can change its color.

1. Open a file.

2. Right-click anywhere in the working space to bring up the pop-up below.

3. Try some other shade of gray. You can also bring up the Color Picker by clicking on "Select Custom Color".

NOTE:  If you want a shade of gray similar to the one in Elements 12, try #6e6e6e.

EDIT: I don't know if you want to go this way but you can hide the bounding box for the Move Tool. Just turn off "Show Bounding Box" in the Tool Options area.