How to connect new wires to battery terminals

It looks to me like that lavender plastic part is some type of fuse to protect the electrical system. You could use the existing bolt or change it to a slightly longer bolt, however, I suggest that you use a 12 gauge wire to a master fuse block (purchase separately), and also install a "master" fuse in front of the new fuse block near the battery. That way, if something should happen in front of the fuse that comes with the light, you will have double protection. I suggest something like this one on eBay (6-Way Car Marine Waterproof Fuse Box Block). When it comes to automotive electrical connections, you can never be too safe with fuses. I have seen cars go up in flames, because of poor wiring by people that had no idea of what they were doing. Every hot wire should be fused.

You can remove the nut, add the 'ring' connector,flat washer, then replace the nut. The flat washer keeps the nut from chewing into the ring connector.

Ideally the new wiring harness has it's fuse close to the battery ! That way it'll blow should the new wiring cut into the frame somewhere along the way to the LEDs.

I plan to add lighting myself, just collecting parts. I'll put a ring terminal on that bolt on the positive terminal with a short wire going directly to a fuse about a foot away from the battery. My output will go from there to a relay. The relay will only energize my lighting power when the key is on. I'll get this relay control from the work light circuit. It may be overkill but it should protect everything.