[wrCisco]

I uploaded a new version of the plugin that should handle the 'quiet' part.
In order to activate the new functionality, you need to manually update the preferences file in $SIGIL_PREFS/plugin_prefs/cssRemoveUnusedSelectors/cssRemoveUnusedSelectors.json, adding the line

"quiet": true

E.g.

{
  "parseAllXMLFiles": true,
  "indent": "    ",
  "indentClosingBrace": true,
  "keepEmptyRules": true,
  "omitLastSemicolon": true,
  "omitLeadingZero": true,
  "formatUnknownAtRules": false,
  "linesAfterRules": "\n",
  "quiet": true
}

After that, it should only interact with the user when there are errors that stop the execution. Let me know if it works okay.