For those who prefer to use the KDE desktop with the theme of Oxygen on their workstations, it was somewhat shocking that the designers of LibreOffice removed the theme of the Oxygen icons from the assembly. The decision they took a year ago, but the hands reached only now in LibreOffice 5.4. Why did they decide that if they do not use the theme, nobody should use it, I do not know why. But the fact is: they made a choice for us. It is not difficult to restore the theme now, it will be a little more difficult in a year, but it's not pleasant. A bit of theory and "practice" below in the article.
"The mind is not a vessel to be filled but a fire to be kindled." — Plutarch