How to add a Select This choice to attributes?

[Patd265]

In Shadows theme, the attributes drop down (size or color etc) defaults to the first variant choice.
(no defaults are actually set)
See example linked below.
How can I change this behavior to default to a line that says Select Size or whatever we need to call it?
https://www.cyclebitz.com/RST-S1-Mes...lack-103465BLK