Short update, but it might make some people happy – the Office 365 Admin Center now features a Dark theme/mode. You can toggle it by navigating to the Home page (https://portal.office.com/adminportal/home#/homepage) and clicking the corresponding link on top (the toggle can only be found on the Home page). Here’s a sneak peak:
Sadly, the Home page seems to be the only one that actually supports this, for example this is how the Groups page will look like with Dark mode toggled:
I’m sure other parts of the portal will be updated in due time. And I’m sure it will be appreciated by the likes of me, that occasionally open the portal on a large monitor during night time.