Foundation Default Mixins are not changeable

[timbergfreund]

I want to change the form-input-error mixin to get a custom background-lighten value. I've tried to add both, "background-lighten": "1" and
"form-input-error": { "background": "blue", "background-lighten": "1" }
to the theme.json, as provided in the example at css-modules-custom, but no changes where shown. A simple "input-background-invalid": "blue", does change the border color, but results in a default 10% background-color for the input field.

Is it possible to change the background-lighten value?