You can still wear a sweater in the summer! As long as it's light material, you won't suffer from heat stroke or anything. Also, cropped sweaters are very popular right now. And who says you can't wear skulls and studs in the summer? As Nike used to say (do they still say it?): Just do it!
Here, I'm wearing a cropped sweater (though it's still pretty long since I'm short) from Topshop, shorts from American Eagle, and my beloved chucks.
Gray Cropped Sweater (Free People)
Black and Green Cropped Sweater (also seen on Aria Montgomery)
Cropped Sweaters (Urban Outfitters)