It is not possible to properly set $link-color variable in user-land config.

Created by: dilame

In your _variables.scss

$link-color:                theme-color("primary") !default;

Your docs (https://getbootstrap.com/docs/4.1/getting-started/theming/#variable-defaults) says

However, when overriding across Sass files, your overrides must come before you import Bootstrap’s Sass files.

But it is impossible to just change $key for theme-color() function, before import _variables.scss, because theme-color expects $theme-colors that declared in _variables.scss. It turns out a vicious circle. I know i can just use a variable instead of function call, but it is a bit confusing. Why you use theme-color("primary") instead of just $primary?