title: Text color not working in Material-UI Theme
body: <p>When creating a theme for colors with Material-UI, I set contrast text to white (#fff). It is working for the button with primary color, but not secondary.</p>
  
  <p>Tried overrides as described here: <a href="https://stackoverflow.com/questions/47813481/material-ui-unable-to-change-button-text-color-in-theme">Material UI: Unable to change button text color in theme</a>. If an override will solve it, then I need help writing one.</p>
  
  <pre><code>const colortheme = createMuiTheme({
      palette: {
          primary: { main: '#e91e63' },
          secondary: { main: '#03a9f4' },
          contrastText: '#fff',
      }
  });
  </code></pre>
  
  <p>Expecting both buttons to have white text. Instead got one button black:</p>
  
  <p><a href="https://i.stack.imgur.com/YovT3.png" rel="noreferrer"><img src="https://i.stack.imgur.com/YovT3.png" alt="enter image description here"></a></p>
  
  <p>Edit: I created the theme and rendered Material UI's SimpleModal component on the parent, passing theme props to the child. The button is rendered on the child.</p>
  
  <p>parent:</p>
  
  <pre><code>const blues = createMuiTheme({
      palette: {
          primary: { main: '#00e5ff' },
          secondary: { main: '#2979ff' },
          contrastText: '#fff'
      }
  })
  
   &lt;SimpleModal label="content" theme={blues} color="primary" document="content" /&gt;
  </code></pre>
  
  <p>child:</p>
  
  <pre><code>            &lt;div&gt;
                  &lt;MuiThemeProvider theme={this.props.theme}&gt;
                      &lt;Button className={classes.margin} variant="contained" color={this.props.color} onClick={this.handleOpen}&gt;{this.props.label}&lt;/Button&gt;
                  &lt;/MuiThemeProvider&gt;
                  &lt;Modal open={this.state.open} onClose={this.handleClose}&gt;
                      &lt;div style={getModalStyle()} className={classes.paper}&gt;
                          &lt;Typography variant="h6" id="modal-title"&gt;{this.state.reference}&lt;/Typography&gt;
                          &lt;Typography variant="subtitle1" id="simple-modal-description"&gt;{this.state.content}&lt;/Typography&gt;
                      &lt;/div&gt;
                  &lt;/Modal&gt;
              &lt;/div&gt;
  </code></pre>