>>56242373Simple. her fingernails are painted black, so naturally you'd think her toenails are black too, but we can't really see that with her game model or anime appearance, can we? hence foot art. And we now know she canonically does thanks to Mastersex's summer outfit.
Also, black toenails are just a turn on in general, at least if the girl wearing looks good with them, like Marnie :D
