title: Material UI Tab label font-size is really small
body: <p>I started to use <a href="https://material-ui.com/demos/tabs/" rel="noreferrer">material ui tabs</a> and I'm having issues with the font size of the Tab labels because they are really small...</p>
  
  <p><a href="https://i.stack.imgur.com/OESus.png" rel="noreferrer"><img src="https://i.stack.imgur.com/OESus.png" alt="enter image description here"></a></p>
  
  <p>This is my code:</p>
  
  <pre><code>import React from 'react';
  import PropTypes from 'prop-types';
  import { withStyles } from '@material-ui/core/styles';
  import AppBar from '@material-ui/core/AppBar';
  import Tabs from '@material-ui/core/Tabs';
  import Tab from '@material-ui/core/Tab';
  import Typography from '@material-ui/core/Typography';
  
  function TabContainer(props) {
    return (
      &lt;Typography component="div" style={{ padding: 8 * 3 }}&gt;
        {props.children}
      &lt;/Typography&gt;
    );
  }
  
  TabContainer.propTypes = {
    children: PropTypes.node.isRequired,
  };
  
  const styles = theme =&gt; ({
    root: {
      flexGrow: 1,
      backgroundColor: theme.palette.background.paper,
    },
    tabRoot: {
      backgroundColor: theme.palette.background.paper,
      flexGrow: 1,
      color: 'black',
      fontWeight: 'bold'
    }
  });
  
  class SimpleTabs extends React.Component {
    state = {
      value: 0,
    };
  
    handleChange = (event, value) =&gt; {
      this.setState({ value });
    };
  
    render() {
      const { classes } = this.props;
      const { value } = this.state;
  
      return (
        &lt;div className={classes.root}&gt;
          &lt;AppBar position="static"&gt;
            &lt;Tabs className={classes.tabRoot} value={value} onChange={this.handleChange}&gt;
              &lt;Tab label="Item One" /&gt;
              &lt;Tab label="Item Two" /&gt;
            &lt;/Tabs&gt;
          &lt;/AppBar&gt;
          {value === 0 &amp;&amp; &lt;TabContainer&gt;Item One&lt;/TabContainer&gt;}
          {value === 1 &amp;&amp; &lt;TabContainer&gt;Item Two&lt;/TabContainer&gt;}
          {value === 2 &amp;&amp; &lt;TabContainer&gt;Item Three&lt;/TabContainer&gt;}
        &lt;/div&gt;
      );
    }
  }
  
  SimpleTabs.propTypes = {
    classes: PropTypes.object.isRequired,
  };
  
  export default withStyles(styles)(SimpleTabs);
  </code></pre>
  
  <p>Is there any way to increase the size of those labels?</p>