title: Changing color of material ui tab label
body: <p>I tried to change the label color of tab to black but it doesn't change. Label color remains white only. Is it hard coded in material-ui only? If no, then how can I change it? This is what I tried</p>
  
  <pre><code>const styles = {
      tab: {
          padding: '2px 34px',
          width: '140px',
          height: '72px',
          color: '#4b4b4b'
      },
      tabItemContainer: {
          background: 'none'
      }
  }
  &lt;Tabs
                          tabItemContainerStyle={styles.tabItemContainer}
                          value={this.state.selectedTab}
                          onChange={this.handleTabChange}&gt;
                          &lt;Tab
                              value={0}
                              buttonStyle={styles.tab}
                              label='Members'/&gt;
                          &lt;Tab
                              value={1}
                              buttonStyle={styles.tab}
                              label='Permissions'/&gt;
                      &lt;/Tabs&gt;
  </code></pre>