Skellige tunic is light armor in The Witcher 3: Wild Hunt, which can be found in Yennefer's room at The New Port inn at Kaer Trolde Harbor. It is also sold by Elihal, Sjusta and the Kaer Trolde citadel armorer.
Unclear why, but Yennefer only approves the Elegant Skellige shirt as part of Geralt's outfit, even though she also provided this as an option for him.