This Safari extension applies darkmode for websites

This extension utilizes CSS filter property. It inverts the whole page. Images and videos are inverted back. Due to this, some colors may then be inaccurate.

You can enable/disable dark mode per site basis.

Thanks to @yep, you can now download it from the App Store

I don't have a Developer ID and can't distribute macOS apps, so the best way is to build it yourself.

Moreover, Safari Extensions seems to be broken in macOS Catalina. You might not be able to enable extension in prefenrences. One workaround is to boot your mac in Safe Mode, enable it, then boot normally.

If you see the following message:

Safari detected an app or service that interfered with clicking

then you can try to enable it manually. Please refer to this guide.