One coming feature that appears to be missing in Windows 10 Technical Preview is a way to toggle between light and dark themes as you can now in Windows Phone. But as it turns out, at least one universal app in WTP2 already supports this functionality. And here’s how you can toggle it on the fly.
This ability—toggling between the light and dark theme on the fly—is new to Windows 10. In Windows Phone 8.1.1, you can toggle between light and dark backgrounds, but doing so changes the look and feel of the entire OS. (Though some apps, like Mail, do offer per-app color choices too.)
But in Windows 10, it appears that you will be able to toggle between the light and dark themes on the fly on a per-app basis. To see this, launch the Store (Beta) app in WTP2. But default, it launches with the light theme that is hard-coded into this release.
To see the dark version, type CTRL + T. Voila.
I wouldn’t expect legacy Modern apps (Xbox Music, Calendar and so on) to offer this capability in WTP2; they will of course be updated over time. But I tested this with other universal apps like Xbox and Photos (both of which are hard-coded to dark right now) and couldn’t duplicate this elsewhere.