I had the same problem. It’s been a while but I’m fairly certain the problem can be solved by installed the appropriate integration package. Try installing these packages, libreoffice-plasmalibreoffice-qt5libreoffice-style-breeze. Take a look at this forum thread for more info, https://discuss.kde.org/t/libreoffice-does-not-honour-kde-theme/3347
I had the same problem. It’s been a while but I’m fairly certain the problem can be solved by installed the appropriate integration package. Try installing these packages,
libreoffice-plasmalibreoffice-qt5libreoffice-style-breeze. Take a look at this forum thread for more info, https://discuss.kde.org/t/libreoffice-does-not-honour-kde-theme/3347