First of all, the style isn't only light and dark but even if it were: yes and no.
We'll use a simpler case than tron to describe the point (because tron is a vastly more complicated case study)
Namely tetris.
specifically the act of cloning the visual or gameplay aspects of tetris.
The “wholesale copying” of Tetris was troubling to the court, which found that the Tetris design, movement, playing field dimensions, display of “garbage lines,” appearance of “ghost” pieces, color changes and automatic fill-in of the game board at the end of the game (all of which were copied by Xio) were aesthetic choices, and were protected, original expressions of an idea. While the idea of a game that required one to rotate figures into a field was not protectible, the design of the component parts was. The court found that the overall look and feel of the games were nearly identical and that any differences between the two were “slight and insignificant.” The court concluded: “There is such similarity between the visual expression of Tetris and Mino that it is akin to literal copying. While there might not have actually been “literal copying” inasmuch as Xio did not copy the source code and exact images from Tetris, Xio does not dispute that it copied almost all of visual look of Tetris.”
read more:
http://www.wired.co.uk/news/archive/201 ... one-ruling
http://arstechnica.com/gaming/2012/06/d ... ng-clones/
Or in other words, significant visual similarity is akin to literal copying to such an extent that in court you could possibly lose.
If we look at the case here: the first included image showcases a character that has striking similarities to the characters in TRON LEGACY, specifically in the dark suit, helmet, and glowing LED parts - but also more subtle details like the specific angles, the fact that there's a protruding ring on the back as well as the light disk in the characters hand in image #2.
This specifically is why I personally will not download it as I can't make any use of it what-so-ever, however your chances of getting sued are nonexistent to minimal (given that it's GPL'd they'd have to send a cease and desist or DMCA takedown notice first and only if you ignore that can you get sued).
Or to simplify this further: Yes, you are probably in copyright violation, however you may also be covered under fair use due to it being a nonprofit derivative art project (that's how I'd legally define it) - but this protection only really extends to you, I can't utilize it in any real way.
And yes, it's crazy - welcome to the world of US copyright law, trademark law, and patents.
as for sig: in this context I'd wager it'd be signature (sig is typical forum shorthand for signature, or the small piece of text that appears under every post a user makes - you currently do not seem to have one, refer to the hexadecimal text under my post for a visual idea of what a sig can look like - feel free to decode it as well, it's really only hex encoded ascii anyway).