title: Howto add PostCSS plugin to the Gatsby SASS plugin
body: <p>I have a Gatsby site with sass support using the <code>gatsby-plugin-sass</code> plugin. That works, but now I want to add PostCSS support.</p>
  
  <p>According to the <a href="https://www.npmjs.com/package/gatsby-plugin-postcss-sass" rel="nofollow noreferrer">warning on this page</a> (a deprecated plugin) this now should be possible by defining the postcss plugin in the <code>postCssPlugins</code> options.</p>
  
  <p>Indeed, the <a href="https://www.gatsbyjs.org/packages/gatsby-plugin-sass/" rel="nofollow noreferrer">sass plugin documentation</a> tells me I can add a postcss plugin to the options, but it's unclear to me how to do this exactly. I already added the <code>gatsby-plugin-postcss</code> plugin separately and am now trying to integrate it with the sass plugin.</p>
  
  <p>This does not work:</p>
  
  <p><strong>gatsby-config.js:</strong></p>
  
  <pre><code>    // SASS support with PostCSS support:
      `gatsby-plugin-postcss`,
      {
        resolve: `gatsby-plugin-sass`,
        options: {
          postCssPlugins: ['gatsby-plugin-postcss'],
        },
      },
  </code></pre>
  
  <p>I guess I should call it in a different way but I can't find any documentation on this?</p>