The next major update to Windows 10 will include this new light theme. The software giant just added it to a new test build of Windows 10, and in a blog post, explained that it's heard feedback from users who want "a truer separation" between light and dark in Windows 10.
Here's how it explained the new theme change:
When you select Light under Settings > Personalization > Colours, the expectation is that the system colour would be lighter too. And it didn’t do that before – the taskbar and many other things stayed dark. Now, if you choose Light under Settings > Personalization > Colors, all system UI will now be light. This includes the taskbar, Start menu, Action Center, touch keyboard, and more.
So, if you are a Windows Insider tester, you can update to this build, and while your system colour won’t automatically change to the new light system colour, you can choose to try the full light experience. Just click the “Choose your colour” dropdown and select Light.
To go along with the new light theme, Microsoft is also changing the default wallpaper of Windows 10. You can use it on your PC by going to Settings > Personalization > Themes and selecting “Windows Light”. This will also change your PC to be light themed.
This new light theme will officially appear in the next major update to Windows 10, expected in April 2019. It's currently codenamed 19H1.