I'm almost 50 and I know what Bratstyle is!
... it's actually a Japanese company that does Bobbers and Bobber/Tracker hybrids that are largely their own unique "Japanese spin" on the Bobber craze ... which has become known as "Bratstyle" ... Bikes modded to imitate their look (Bobbers with bench seats instead of a solo is one of their signatures ... so are black cast wheels intead of traditional spokes) are considered Bratstyle or Brat Bikes ...
Here's a W650 that has been "Bratted";
And another Brat Bobber ... not sure of the make ...
Here's a Bratstyle Sportster;
Brats are VERY cool!