The test build 19H1 (also known as 1903) was released three weeks ago to skip ahead testers and is supposed to be fully rolled out by April 2019.
But, on September 6, Microsoft released Build 18234, which had a few updates and support for a few related app refreshes. The release is quite early referring to previous to update cycle and so lacks new big features.
This 19H1 build had dark theme support for File Explorer the feature already present in Windows 10 October 2018 Update test builds.
Snip & Sketch, initially known as Screen Sketch, also a snipping in windows 10 had improvements to so that the “New” button is now working.
Microsoft’s To-Do app now includes inking support. Sticky Notes the latest version of which is 3.0 adds syncing notes across devices.
XAML shadows, ability to name the group of tabs and acrylic in some pop-up controls are one such feature Microsoft is removing previous releases in today’s 19H1 test build temporarily. Microsoft says these will return on a future flight.
A full list of updates, fixes and improvements is listed in the September 6 blog post about Build 18234.