Code highlighting

[thomasheartman]

Hi,

Thanks for the work on the package; it's the one thing that made it a viable choice for me to switch over to Gatsby, so I'm very grateful.

I've got a question about code highlighting. In the example, the first JS example is highlighted. The gatsby-config file in this repo also seems to indicate that highlighting should be automatic:

        // if you don't want to have server side prism code highlight
        // noHighlight: true,

However, it doesn't seem to work for me, so I'm wondering how it's supposed to work? I tried both with and without :exports code, with rust, toml, and js, but there's no highlighting going on.

I was able to make prismjs work by using the remark plugin as below, though:

    {
      resolve: `gatsby-transformer-orga`,
      options: {
        plugins: [
          {
            resolve: `gatsby-remark-prismjs`,
            options: {
              aliases: { sh: "bash" },
              showLineNumbers: true,
            },
          },
        ],
      },
    },

but the root pre tags don't have the right class attached, so I need to manually set their background color. It's a hack, but it works.