I had a vague idea that there were women that dressed like this (and I saw that Simpsons were Marge joins them, but the turn out to be a team of thieves), but I had no idea what the point/philosophy was. Since the outfit isn't iconic like Santa, Brides, pirates, ninjas, etc, I didn't think we were witnessing some kind of costumed bar crawl.
Finally, I got it:
"They're an all-woman Linux users group!"
dirtylibrarian thought not.
disambiguation for those who don't get why that was hilariously funny: