inverted color opaque fallbacks should get 'darken' instead of 'lighten'

e.g.

```scss
// wrong (current):
$C_lineGrayInverted: rgba(255,255,255,0.2);
@mixin color_lineGrayInverted($style: 'color') { #{$style}: lighten( rgb(255,255,255), 80%); #{$style}: $C_lineGrayInverted; }

// better
$C_lineGrayInverted: rgba(255,255,255,0.2);
@mixin color_lineGrayInverted($style: 'color') { #{$style}: darken( rgb(255,255,255), 80%); #{$style}: $C_lineGrayInverted; }
```