What I do is this:

on helpers.scss

@use 'variables' as vars;

on _variables.scss

$black: #000;
$white: #fff;

$colors: (
  'black': $black,
  'white': $white,
);
:root {
  @each $name, $color in $colors {
    --#{$name}: #{$color};
  }
}

and I get on main.css

:root {
  --black: #000;
  --white: #fff;
}