Hi @svenvandevelde.
Cool project!
The most important thing to understand is that Arduino IDE 2.x extensions are VS Code extensions. Any information you find about writing VS Code extensions is applicable to making an extension for Arduino IDE 2.x. VS Code is tremendously popular so you will be able to find plenty of information about making VS Code extensions. You can get started here:
Yes. Starting from Arduino IDE 2.2.0, the IDE comes with a VS Code extension named "VS Code Arduino API" preinstalled. This extension makes the Arduino state data available to all other extensions.
You can learn about this from the extension's documentation:
https://github.com/dankeboy36/vscode-arduino-api/tree/main#readme
You can use the source code of this Arduino IDE extension as a reference model:
as well as this one:
Cool project!
You should select "New Color Theme".
There are some additional instructions for making a custom Arduino IDE 2.x theme extension here: