@PeterJones Thank you very much for such detailed explanation. You are right the crux of the problem was the fact I confused what was N++ functionality and what was that of the plug-in (you know how it goes, I installed that plug-in many years, it all just worked with every N++ update, so I did not give a thought to it).
Upon realising my mistake, I was able to fix it myself using information on the page of the manual you referred to (the xml-file was in the old format, in deed, so the icon set look got back to what I want it to be as soon as I edited the file in accordance with provided guidelines).
Thank you again, you have been most helpful!