title: Material-UI Changing default color
body: <p>How can I change the default color ? 
  What is the object I need to modify in the theme.js ? </p>
  
  <p>EDIT </p>
  
  <p>I want to modify the defaut (grey color) who isn't <code>primary</code> or <code>secondary</code> or <code>error</code>. </p>