SASS functions are available in the customize theme section, it’s compiled when you save your modifications. Your can’t use SASS functions on theme or CSS variables though.
If you want to use a SASS function on a color from your color scheme, you must use the corresponding SASS variable.
Will work 
body {
background: darken($secondary, 10%);
}
Won’t work 
body {
background: darken($my_theme_variable, 10%);
}
Won’t work
body {
background: darken(var(--secondary), 10%);
}
And on the paper, I think what your link describes should work. 