You can change them just like any other thumbnail in Inspector > General.
- Copy an image
- Click the thumbnail in the Inspector
- Paste
If you want to colorize them (instead of replacing them with an image) use this Script: Colorize DEVONthink Icons.
To use it you need to install ImageMagick, e.g. via Homebrew.
As I can tell from your previous posts you really like your DEVONthink with custom colors, so I think this script is exactly what you’re looking for as it allows to colorize any DEVONthink record (groups, smart groups etc.).