The dark system theme I have set seems to make some applications look funny, with dark boxes where they should be light. Is there any way I can manually set a theme on a per-application basis?
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-plasma libreoffice-qt5 libreoffice-style-breeze. Take a look at this forum thread for more info, https://discuss.kde.org/t/libreoffice-does-not-honour-kde-theme/3347