Hi @llaves. You can select between four pre-installed themes by doing this:
Select File > Preferences from the Arduino IDE menus.
Pick the theme you want from the "Theme" menu.
Click the OK button.
But I suspect that maybe you are not satisfied with the existing themes and want to make a custom one. There is some information about this here:
So it sounds like this is no official support for doing this currently, but plans to provide it at some point.
I suspect that there is a way to modify the themes, but not in a very user friendly manner. The IDE framework Arduino IDE 2.x is built on is Eclipse Theia, so any relevant information you find through research about Theia will likely be applicable to Arduino IDE 2.x as well. If you do investigate this, it would be great if you'll report back here with your findings. I'm sure other forum members would be interested to learn more.