Good afternoon, @Tom_Meighan. 

Thanks for stopping by on this thread. 

The option to switch QuickBooks Online to Dark Mode, used to be a feature in the system. This was one of the beta features we have for QuickBooks Labs. We've recently removed the QuickBooks Labs and this includes the Dark mode and QuickBooks Themes. 

In the meantime, you may consider checking your browser's settings on how to set it up with Dark Mode.

I hope this helps answer your question. If you have any other concerns, don't hesitate to ask. Take care!