This is the repository of the Arduino IDE snap.
This snap includes the Dark Arduino Theme as an option. You can enable it in File > Preferences > Theme.
If you want to add a different theme, you can do so by placing the .zip file of the theme in the folder ~/snap/arduino/current/Arduino/theme. You might need to create this folder first. The .zip file needs to include all the files in the theme folder of your theme. Make sure to zip the contents, not the directory itself, so the theme files are in the root of the zip instead of in the theme folder.