rickvanlieshout.com/gatsby-ssr.tsx

import { RenderBodyArgs } from "gatsby";
import React from "react";

const setColorTheme = `
(function() {
  const mode = localStorage.getItem('theme');
  if (mode !== null && ['light', 'dark'].includes(mode)) {
    document.documentElement.dataset.theme = mode;
    return;
  }

  const preferredColorScheme = window.matchMedia('(prefers-color-scheme: dark)');
  const hasMediaQueryPreference = typeof preferredColorScheme.matches === 'boolean';
  if (hasMediaQueryPreference && preferredColorScheme.matches === true) {
    document.documentElement.dataset.theme = 'dark';
  } else {
    document.documentElement.dataset.theme = 'light'
  }
})();
`;
export const onRenderBody = ({
  setPreBodyComponents,
  setHeadComponents,
  pathname,
}: RenderBodyArgs) => {
  const currentUrl = `https://www.rickvanlieshout.com${pathname}`;

  setHeadComponents([
    <meta data-url="currentUrl" key="og:url" property="og:url" content={currentUrl} />,
    <link data-url="currentUrl" key="canonical" rel="canonical" href={currentUrl} />,
    <meta data-url="currentUrl" property="test:rick" key="test:rick" content={currentUrl} />,
  ]);

  setPreBodyComponents([
    React.createElement("script", {
      key: "theme",
      dangerouslySetInnerHTML: {
        __html: setColorTheme,
      },
    }),
  ]);
};