To make a skin for Light Cycle 3D v3.0, simply make a new folder with the name of your skin.
Inside this folder must be the following images in their respective formats:

back.png
bottom.png
front.png
left.png
right.png
top.png
floor.png
wall_1.png
wall_2.png
wall_3.png
wall_4.png

Then simple go to the options in-game and select your skin and have fun!