It’s possible with custom meta data and two smart rules. One searches for groups, one searches for smart groups whose custom meta data “icongeaendert” is 0.
Set up your custom meta manually and set it to 1 for Inbox , Annotations and Tags as you probably don’t want to change these icons.
Only tested it on a test database (not on all databases and the global inbox), so maybe I’ve missed something…
-- Change icons
property iconPath : "/Users/user/Desktop/Test.png"
on performSmartRule(theRecords)
tell application id "DNtp"
try
repeat with theRecord in theRecords
set thumbnail of theRecord to iconPath
add custom meta data 1 for "icongeaendert" to theRecord
end repeat
on error error_message number error_number
if the error_number is not -128 then display alert "DEVONthink" message error_message as warning
end try
end tell
end performSmartRule