LEDs used specifically for decoration must adhere to all robot rules. Specifically but not all inclusive, the LED must not be of such a color that it would confuse other robot’s vision systems or interfere with human vision in the players station. All electrical rules also apply, insulation, wire size and breaker choice.
Lights would be cool, but I doubt that there will be any room on a finished robot for very much decoration. I would focus on strategy first and then the fancy stuff. But if you can, make sure that the lights are not distracting. Team decals and logos are good too.