Copyright to your work is established immediately when your work becomes fixed in its ultimate form. You don't have to worry about losing it.
Assuming that your site existed before theirs, they MAY be found to be infringing. Emphasis on "may" because you don't have a copyright on ideas, only on the expression of the ideas, and it has been ruled that the "look" of a site isn't protected. So, for a court to rule that the they are infringing, it will have to find that significant portions of the copy, code, or presentation must be a near duplicate.
I haven't viewed the two source files or CSS, so I can't even an opinion on the significance of the replication in this case.
Regardless, there are several reasons why pursuing this in court may not be a good idea. First, your copyright isn't registered, so you have the burden of proving that your site existed first. To do this you'll have to meet with an attorney several times, file to have their ISP provide evidence that you will need to prove you case, suffer through the other side's discovery, depositions, etc. Basically, it will waste a lot of time. Second, even if you win you will have to show that you suffered some loss due to the infringement, and any monetary award will be based that loss. You're loss would likely be found to be the percentage of their revenue derived from infringing on your copyright. It appears that in this case, that would be negligible.
All this being said, it wouldn't hurt to have a lawyer write a cease and desist letter, and another if necessary. At the very least, that will force them to reword there sales copy, and rework their markup.