Sentence A sounds fine to me.
In sentence B, the phrase “very in” is unnatural to me. You can remove the “very” and just say “wearing T-shirts back to front is in these days”, but that sentence could be confusing because people may not realize that “in” is short for “in fashion”. I think it is better to say:
“…wearing T-shirts back to front is very fashionable these days”
“…wearing T-shirts back to front is in fashion these days”
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