fix codeblock theme on landing page

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for instance see video below

Bildschirmaufnahme.2021-02-22.um.14.05.15.mov

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facebook/docusaurus#4258 (comment)
might have something todo with (see above)

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// The Prism theme on SSR is always the default theme but the site theme
// can be in a different mode. React hydration doesn't update DOM styles
// that come from SSR. Hence force a re-render after mounting to apply the
// current relevant styles. There will be a flash seen of the original
// styles seen using this current approach but that's probably ok. Fixing
// the flash will require changing the theming approach and is not worth it
// at this point.
https://github.com/facebook/docusaurus/blob/master/packages/docusaurus-theme-classic/src/theme/CodeBlock/index.tsx