Earlier this month, we reported that a Dark Mode was coming to Outlook.com. Well, it is finally here and currently being slowly rolled out to Outlook.com users right now.
Dark Mode is currently only available in the beta version of Outlook.com, so be sure to be using the beta before checking to see if Dark Mode is available for your account. To check if Dark Mode is enabled in Outlook.com, go into Settings > Quick Settings > Dark Mode.
If the Dark Mode toggle is there, just toggle it to the On position and Outlook.com will be shown in the new Dark Mode theme shown below.
There is one caveat, though, to using Dark Mode, which is that not all themes will be available when using it. According to the information bubble, "Some themes aren't available in Dark Mode", which makes sense as some themes use colors that would not be properly visible in the darker theme.
If you do not see the Dark Mode toggle yet in Outlook.com, you will just need to be patient. This is not a region based rollout, but appears to be either a targeted test, A/B test, or gradual rollout so it may take some time to before it becomes available for everyone.