Investigate and try out modern Swing look and feel, dark theme

[bjornregnell]

Swing has this cool look-and-feel, esp Dark theme etc.:
https://www.formdev.com/flatlaf/
Investigate how to use this and check that it works on WSL, Windows 10, Windows 11, macos, Ubuntu...
Impacts the Swing.scala file.
The latest Kojo release use it, so in that code-base there should be inspiration to make it platform-independent.