Translation:In the summer they wear short skirts.
In my verb dictionary, I found: Lei porta sempre dei begli abiti (She always wears beautiful clothes). No specific clothes are referred to, as in this case, the "short skirts"
In another thread, someone mentioned using "dei" (in that case) "as the plural of the indeterminate article "un"" Thread found here: https://www.duolingo.com/comment/1355622
Examples in Collins Italian dictionary: portava un bel vestito (she was wearing a beautiful dress) and non porto più queste scarpe (I don't wear these shoes any more). In both the Collins cases without the variants of di, they refer to specific items worn.
If you consider something, whether it is short skirts or whatever, as being a subset of the set of all possible items in that category then you can usually use 'di', either by itself or inflected with the definit article, in Italian. This is regardless of whether English use an explicit 'some' or 'any' or not.
But they do wear some short skirts - not all of them, which would be the implication if one said «… le gonne corte». Think about saying this about one person. "She wears a short skirt." This has an singular indefinite article. It therefore stands to reason that, with more than one person, and therefore skirt, you would have a plural indefinite article - that's the partitive, «delle».
Using a definite article, as in «le gonne corte», conjures the implication of short skirts in general, and so would be understood as every short skirt in existence. Not ideal.
As for no article at all, just «gonne corte», I think that would be okay? I'm not confident though.
Apropos nothing, a better verb here would be «indossare». Portare has enough meanings even without meaning "wear" as well.
Here's a good article: https://www.thoughtco.com/italian-partitive-articles-2011451