Nice icons. If you can post the source, others might enjoy using them.
It is easy to change the icon for a group or document. Open Tools > Show Info. Click the icon in the upper left corner. If you’ve copied an icon from elsewhere onto the clipboard, you can paste it there. Or, you can copy the icon from one group or file, select a different group or file (Show Info will change as you change which thing is selected), and paste it to the second group or file. Or, if the item is indexed, you can change the icon in the file system using Finder’s Get info dialog in a similar way to Show Info – refresh the indexed files in DEVONthink and the new icon will show up in DEVONthink.
In both cases, Finder’s Get Info and DEVONthink’s Show Info, if you select an icon and delete it, the default icon for that group/folder or file is restored.