Arduino IDE 2.x uses a different theme system. It supports the use of VS Code themes. This is very good news if you are hoping to find a pre-made theme because the tremendous popularity of VS Code means there are thousands of free 3rd party themes available. That is a significant difference from the less than 20 3rd party themes that are available for Arduino IDE 1.x (many of which are unmaintained).
The instructions for finding and installing 3rd party themes are provided here in the advanced usage documentation for Arduino IDE 2.x:
https://github.com/arduino/arduino-ide/blob/main/docs/advanced-usage.md#3rd-party-themes
In case you can't find a pre-made theme that meets your needs, or if you just want to play around with the theme system, it is possible to make your own custom themes. A community member shared some information about doing that in this post: