This boilerplate is intended to be used in a "mobile-first" context, where the mobile version is also shown to browsers that don't support CSS GRID and CSS VARIABLES.
The idea is to have a robust mobile version that can work everywhere, but nothing prevents you from enhancing it with modern features through @supports.
A predefined @supports query string is available in _variables.scss ($supports-string).
@supports (#{$supports-string}) {
display: grid;
}