How to apply fontSize to CardHeader title in Material UI?

I want to change the title in CardHeader to 16px. I tried changing theme in App.js but it does not seem to work

const theme = createMuiTheme({
  typography: {
    useNextVariants: true,
    overrides: {
      MuiCardHeader: {
        titleTypographyProps: {
          variant:'h2'
        }
      }
    }
  }
);

In the component:

<CardHeader
  action={
    <IconButton color="inherit">
      <MoreHorizIcon />
    </IconButton>
  }
  title="Titletext"
/>

The title font still does not change. What do I need to do to fix this?

you cant target the header class or id and change fontSize or pass as props

titleTypographyProps={{variant:'h1' }}

that object acepts:'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'subtitle1', 'subtitle2', 'body1', 'body2', 'caption', 'button', 'overline', 'srOnly', 'inherit', "display4", 'display3', 'display2', 'display1', 'headline', 'title', 'subheading'

in your code it would be

<CardHeader
        action={
        <IconButton color="inherit">
            <MoreHorizIcon />
        </IconButton>
        }
        titleTypographyProps={{variant:'h1' }}
        title="Titletext"
      />