My Windows Outlook 365 menu bar used to look something like this:

[![enter image description here](https://i.stack.imgur.com/1Jene.jpg)](https://i.stack.imgur.com/1Jene.jpg)

As of yesterday, all of a sudden, it has started to look like this:

[![enter image description here](https://i.stack.imgur.com/AYaWP.png)](https://i.stack.imgur.com/AYaWP.png)

I have no idea what I did, or even if I did anything, but the icons in the menu bar have all changed. I've tried changing the Outlook theme. I've spent ~1 hour searching on Google. What happened and is it possible to change it back?