To enable the theme, hit the Settings cog in the top right corner and then move the Dark mode slider to the right:
To do this we’ve been emailing drafts of the chapters to and fro. And (again) this morning a feature of Outlook.com saved me from looking slightly silly when I replied to an email intending to send back an updated draft, but clicked Send without actually attaching the file;
I was presented with the above message. Clicked Go back, added the file and sent the message.
I’ve talked to various people who dislike this type of message, but I really don’t see the problem with it. I’ve only ever been presented the message when I have meant to attach a file and forgotten; I’ve never received it for a false positive so I quite like it. I wish Outlook 2010 had it (or perhaps it does and I just don’t know where the setting is.