gatsby-config-1.js

plugins: [
  ...
    {
      resolve: `gatsby-plugin-postcss`,
      options: {
        // Accepts all options defined by `gatsby-plugin-postcss` plugin.
      },
    },
   {
      resolve: `gatsby-plugin-emotion`,
      options: {
        // Accepts all options defined by `babel-plugin-emotion` plugin.
      },
    },
    ...
  ],