title: Type 'FlattenInterpolation<ThemeProps<any>>' is not assignable to type 'FlattenSimpleInterpolation'
body: <p>Anyone get this with styled components when using typescript? Im trying to use mixins:</p>
  <p>Whenever i use a css tagged template i am told that it is not assignable to type FlattenSimpleInterpolation. Anyone know the correct way of doing this?</p>
  <pre><code>const extraStyles = css`
    background-color: ${props =&gt; props.theme.colors.lightGrey};
  `
  
  </code></pre>