Since the developers of VSCodium keep changing the icon, I've uploaded mine here, together with the Pixelmator template to re-create it. That's it.
To change the icon on macOS
- open the svg file from this repo
- select all & copy the contents to your clipboard
- change to the Application folder on macOS
- click on the VSCodium.app and use File > Get Info
- select the existing icon and paste the contents of the clipboard over it
The new icon will show once VSCodium has been restarted.