That's a great suggestion and I'll definitely bring that up to our software engineer folks to see if that's something we can implement soon.
As a work around for the meantime, you could use a dark mode browser extension to achieve the same effect. Personally I use one on Google Chrome called, "Dark Reader", and it will automatically switch all websites to dark mode. It also allows for manual configuration, so you can choose to disable/enable it for specific websites.
There's a bunch of other Dark Mode extensions with different looks/finishes as well, so you can have some fun with it.
Unfortunately, this depends on if your browser allows for extensions and may be limited choices if it's a browser other than Chrome, so hopefully the website based toggle is something we can add in the near future.