Custom CSS styles & Mixins

I been using your previous version for polymer 1.X and the custom css styles & mixins worked. Now for version2 it doesn't seem to work anymore.

I am using this mixin:
`    --paper-buttons-group-selected: {
          color: #000;
          background-color: #fff;
        }`

Only background-color is applied but not the font color. Is it a bug so something I am doing wrong?

Thanks