add light&dark mode

[whatqiu]

Description

add light&dark mode

Linked Issues

https://github.com/ddiu8081/chatgpt-demo/issues/71

Additional context

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Name	Status	Preview	Comments	Updated
chatgpt-demo	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Mar 7, 2023 at 4:29PM (UTC)

[ddiu8081]

Thanks for your contribution! I'm a little busy on Sunday, I'll check it out and merge it in later, please wait a bit!

[Limour-dev]

Hope the font color in light mode can be black