I am creating a SCSS for a component, and looking at ExtJS SCSS I find in Base.scss that a lot of variables (if not all) are defined as:
Ext uses Fashion, which is an extension of SASS. See: https://docs.sencha.com/cmd/6.x/fashion.html#Dynamic_Variables
It's somewhat similar to
!default in SASS, but it defers computation until all variables are known.