This is a very nice skin, but there are a few things I would personally like to see fixed on it. First, the fact that the same graphic is used on both regular 300 and rainbow 300 is not good, as being able to know when you have perfect timing vs only ok timing is essential in mania. To do this, I would suggest creating a different color of "GOOD", such as how the O2Jam skin by eca2free did with creating the green "good" and using that as 200 while keeping the blue "good" as 100.
Second, in IIDX the judge does not have a kind of "popping" animation like other games, such as O2Jam. If you watch someone like DOLCE. play, you see a solid silver "GREAT" instead of a continuously popping one, so I would try to fix that if possible to make it more like actual Pendual.
Then more as just an aesthetic thing, it might be good to have custom icons for the different mods, although this obviously isn't really all that necessary.
Overall it's a really nice skin, much better than the old "IIDX Mania" one in my opinion, but a few modifications could make it even better
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