Please tell me the country where declining to offer that candidate a job would be illegal.

Australia. It's not clearly illegal but it's dangerous territory. Candidates have a general right to be treated as equals and you need to reject someone for reasons that are relevant to the job position.

Something that can easily be changed, like a shirt, might not be OK. ANZ bank (a massive bank with several hundred billion dollars in assets they manage), for example, requires customer facing staff to wear a branded uniform but back at the office? You can wear whatever you want. When they changed their dress code years ago to no-longer require a suit/tie the CEO deliberately wore ugly clothes for a while to set an example.

Obviously no candidates are expected to turn up to an interview in their uniform - they don't have a uniform yet. And if someone can wear a Marilyn Manson shirt in the office, then why not also at the interview?

The bank I'm with is even more relaxed - even customer facing staff can wear anything they want. Sure, if it's offensive they'll be told to wear something else, but that's a conversation I'd be having with the candidate rather than a reason to reject their application. I might reject them if I don't like their response.