It's long overdue, but Microsoft is finally refreshing the icons on Windows 10. It should be coming to the core apps in the months ahead, starting with the Calendar and Mail apps. Those in the Release Preview for Windows Insiders can already see the new look for these two apps.
According to Microsoft's design team, they aim to break away from the colourless, flat-looking icons to make ones that match the company's branding. At the same time, their goal is to make it easier for you to spot these programs.
Source: Engadget