The name for the upcoming "19h1" update to Windows 10 has made its first appearance. It looks like Microsoft is planning to use the wording "April 2019 Update" as the final name for this release.
Microsoft is releasing a new build from the development branch (the next Windows 10 version, currently known as version 1903, or 19H1). Windows 10 Build 18323 features a number of improvements made to the built-in Light theme, RAW Image format support, and more. Here's the change log.