The TLDR here is Microsoft officials know that Notepad is not an app that needs a lot of change. Today's update is meant to "feel fresh, but familiar," as Microsoft's blog post states.
The biggest news is Notepad now supports dark mode. By default, Notepad will match users' system theme preferences, but users can change the option in the settings page for Notepad, which also includes font options.
The user interface of the refreshed Notepad is meant to align with the visual design of Windows 11, which means it gets rounded corners and Mica, which is a design "material" that incorporates theme and desktop wallpaper to paint the background of "long-lived" windows.
Today's test build also includes a redesigned find and replace feature, as well as support for multi-level undo, which officials said is another top community feature request.
There are known issues in today's release, including issues affecting keyboard access key usage when switching between different input languages or using Japanese IME. Officials said they will continue to work on improving performance when opening very large Notepad files.