feat(docs): support playground sync theme

[btea]

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Sync element-plus/element-plus-playground#265

[github-actions]

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=16735
Please comment the example via this playground if needed.

[kooriookami]

There's an error in dark mode: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.

[btea]

I created this PR so that if the dark mode is selected when viewing the document, it will also be set to dark mode simultaneously when clicking on playground to jump.

I don’t understand a bit about the error you mentioned in dark mode. 🤔

[kooriookami]

Demo

[btea]

Thanks for the demo, I understand.

[kooriookami]

Components still be light.

[btea]

This should be a problem on the playground side. The dark mode code corresponding to the PR-generated style code is not loaded.