How do you pass variables to pageQuery : <p>I have this page in Gatsby:</p>

<pre><code>import React from 'react'
import Link from 'gatsby-link'

import IntroPartial from '../partials/themes/intro'

export default class ThemeTemplate extends React.Component {
  render(){
    const theme = this.props.pathContext.theme
    console.dir(this.data)
    return (
      &lt;div&gt;
        &lt;h1&gt;{theme.name}&lt;/h1&gt;
        &lt;IntroPartial theme={theme} /&gt;
      &lt;/div&gt;
    )
  }
}

export const pageQuery = graphql`
query ThemeQuery($theme: String){
  allMarkdownRemark(
    filter: { frontmatter: { themes: { in: [$theme] } } }
  ){
    edges{
      node{
        frontmatter{
          title
          themes
        }
        html
      }
    }
  }
}
`
</code></pre>

<p>This query works fine in the GraphQL tester assuming I supply an option to <code>$theme</code>. How do I provide the value for <code>$theme</code>? I'd like to set it to <code>this.props.pathContext.theme.slug</code>.</p>

<p>The docs seem to imply that some variables should just work, but I'm not sure how to add my own.</p>
