Microsoft has pushed out Windows 10 Mobile build 10166 to members of the Windows Insiders Program on the Fast ring. The build for the Mobile version showed up after Microsoft seeded the preview update for the Windows 10 PC version.
Installing the latest preview build, you should see a number of small changes and feature tweaks. However, Microsoft is focusing more on the improvements related to the core quality of the operating system.
Microsoft has squashed a majority of the bugs present in the last Windows 10 Mobile build 10149, released at the end of June.
The most notable one is the post-upgrade migration status screen missing issue that crops up after the upgrade. The bug makes the lock screen appear frozen, without the date and time for up to 10 minutes.
With the latest preview, the post-upgrade migration status screen appears normal. Additionally, Microsoft has fixed an issue where Insiders were unable to lock their phones even after entering the PIN correctly several times. This was caused by a TPM issue.
The colours in the apps should now appear correctly including the titlebar in the Outlook Mail app, which is no longer orange.
You should also find improved reliability of app downloads and updates from the Store with the latest build. Do not forget to check the Windows Store to get the latest app updates.