fix: add reactive theme content

[kieranwv]

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In shiki's configuration, I set experimentalThemes: { } as documented, but it doesn't work when I dynamically switch dark mode and light mode.

Then I checked the official documentation of shiki and found that I need to manually set some css fragments to get reactive dark mode.

I replaced the .shiki class name with .astro-code and added the code to my documentation project and now it works fine.

I added this usage to the document, hoping to help developers who are unfamiliar with shiki. 😀