I dress like I would in spring or fall. I don't really differentiate between seasonal clothing, although some items work better across the seasons, of course. I pick my outfit depending on the weather and not anything else. That's why my spring and summer looks are often darker or less "summer like" than others on the forum, but I don't really care.
If I had to recommended one piece of clothing for a cool summer day it would be a wool camisole, I love mine.
Btw, we have central heating and in summer it's turned down almost completely. I have a bit more leeway in the bathroom, but otherwise I have to compensate with my dressing. A few weeks a go I was wearing a tee, shorts and sandals outside and warm socks, long pants and a wool pullover inside (the building is slow to heat up when summer temperatures arrive).