When used to refer to fashion, "Strega" is a catch all term invented by tumblr users to describe outfits usually inspired by the Mori Girl subculture, Dolly Kei, and vintage and gothic clothing. The recent vogue for witchy clothing may have also been influenced by the outfits in American Horror Story: Coven, and the discovery for some younger people of Stevie Nicks.
The label was intentionally left vague- people on tumblr often post daily outfit photos, and sometimes arguments arise- is this minimalist outfit with the bulky black sweater have enough going on that is intentionally dark to be considered goth? What if I normally prefer pastels and confuse The Cure with The Darkness and don't want to try to make it seem like I'm a member of the gothic subculture? What if I'm a goth and I want to experiment with the Mori Girl look, but don't feel comfortable wearing the usual pale and earth tone layers, so I want to wear blacks, greys, and darker floral prints? For some time the latter was a huge subject of debate in the online fashion community, because in Japan, where the subculture and style originated, no one does this, but it looked cool, and the Mori Girl ethos is somewhat live and let live. The term "Black Forest Mori" was considered and then thrown out as potential cultural appropriation, although, since Tumblr is Tumblr, people have discussed whether or not the term is offensive to witches in Italy or not.
The idea is that if your outfit is something you'd describe as "Witchy" you are welcome to tag it strega. Strega is not associated with any particular lifestyle or culture.