AdminLte/src/scss/bootstrap-dark/_root.scss

:root {
  // Custom variable values only support SassScript inside `#{}`.
  @each $color, $value in $colors-alt {
    --#{$variable-prefix}#{$color}: #{$value};
  }

  @each $color, $value in $grays-alt {
    --#{$variable-prefix}gray-#{$color}: #{$value};
  }

  @each $color, $value in $theme-colors-alt {
    --#{$variable-prefix}#{$color}: #{$value};
  }

  @each $color, $value in $theme-colors-rgb-alt {
    --#{$variable-prefix}#{$color}-rgb: #{$value};
  }

  --#{$variable-prefix}white-rgb: #{to-rgb($white-alt)};
  --#{$variable-prefix}black-rgb: #{to-rgb($black-alt)};
  --#{$variable-prefix}body-rgb: #{to-rgb($body-color-alt)};

  // root and body

  --#{$variable-prefix}body-color: #{$body-color-alt};
  --#{$variable-prefix}body-bg: #{$body-bg-alt};

  // ?

  --#{$variable-prefix}gradient: #{$gradient-alt};
}