Rule to change MDI Button

Hi All

Developing my own lathe screen set and wondering if i'm doing something stupid.

Have the following in Style.qss
MDIButton [style='active']{
  color:green;
}
MDIButton [style='nactive']{
  color: rgb(255, 255, 255);
}

And the rule is as per the attached image but it doesn;t work. I;ve done similar for text elsewhere but I can;t get the button color to change.

Any help gratefully received