I don't think so. It seems to me that "Det er" is the equivalent of the English "It is" when used like in these examples:
It is a cat: Det er en kat
It is my cat: Det er min kat.
It is me: Det er jeg.
Even if the "it" you're talking about is a common gender word, you still use "det er", but only when saying "It is...." That's why I think it can only be "Det er mørkt udenfor". Any natives who could clarify?