https://www.theregister.co.uk/2018/07/25/outlookcom_goes_dark/

Goths of the world rejoice! Outlook.com's Dark Mode is here, turning mailboxes into blacks and shades of grey such that Lego Batman would shed a tear of joy. If Lego Batman cried. Which he doesn't.

Microsoft confirmed on Outlook's uservoice forum today that Outlook.com (the beta version of the webmail site at least) could be toggled into the much-requested Dark Mode, switching the selected theme (only the default blue theme at present) into a darker version.