Why aren't gtk color variables used in the Adwaita scss?

[Wayne Rowcliffe <wrowclif gmail com>]

I was looking to extend the Adwaita theme to use some different colors.

This would be very straight-forward if Adwaita used @define-color
variables throughout the theme since I could just redefine those
values. However it looks like the Sass $variables are used instead.

Is there a technical reason why we can't use GTK's variable support?
It seems like it would be good on multiple levels, both because
theme's would be easier to customize and because we would be using our
own features.

-- Wayne Rowcliffe