Invalid CSS from the date picker

When using the date picker we are getting this error from the w3 validator(The W3C Markup Validation Service): ‘Error: CSS: color: #cacccd is not a color value.’

The css causing this is:

.SingleDatePicker_closeButton:hover {↩
    color: darken(#cacccd,10%);↩
    text-decoration: none↩

It seems the date picker isn’t compiling the scss into css correctly as the darken attribute should have been translated into a transparent hex value. But for the life of me I cannot find where in my application I can potentially run it through a scss compiler. Does anyone know where I can make the change?