Themes already have the ability to define sass variables:
However, from personal testing, these seem to be local to the theme’s stylesheets, and won’t globally override a variable.
If they did it would prove incredibly useful for theme development, as it would allow the theme to specify exactly what colour should be used as $primary-medium, $tertiary-low and so on - rather than have it automagically calculated off of the colour scheme.
Perhaps there should be a scope option, something like this: