_lighten_color.scss

// Sass's lighten function just adds the supplied percentage to the existing color's percentage
// making it really difficult to adjust a color's lightness in any sort of logical way. This
// function solves that issue increasing the percentage relative to the color's lightness.
@function lightenColor($color, $percentage) {
    $diff:          100 - lightness($color); // Find the percentage difference
    $quotient:      $diff / 100%;            // Dividing a percentage by a percentage return a decimal
    $newPercentage: $quotient * $percentage;

    @return lighten($color, $newPercentage);
}