Add the ability to switch between white and dark themes

[wudifeixue]

Hi again, I think it would be a pretty nice feature if we can switch the theme black and white by click on the extension icon.

[renatomefi]

@wudifeixue Would be awesome if you could send a PR for that!

[Crecket]

I'll send a pull request in a bit, give me a few mins

[Crecket]

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