How to reuse variables (colors) defined on package.json

I'm using this package to customize the theme, with this:

```
"reactToolbox": {
    "customProperties": {
      "color-primary-dark": "#607D8B"
    },
    "output": "client/"
  },
```

I want to make use of that new `color-primary-dark` color in my own custom `.css` files. There is some way to do that?

Cheers!