Dark mode support

[samwoolertonFosters]

I expect most apps now to support dark mode, as it's pretty easy to do.
Would be great for Papercups chat widget to support this too - I imagine we could configure via the iframe query string, and either just set darkMode=true and you set the dark colour scheme, or we pass in options like backgroundColor, textColor, etc.

I'd fork the repo and implement myself if it wasn't implemented via an iframe.