Last time, I see, that my LibreOffice is sick. Original LibreOffice, which included in standard Kubuntu was not look like that. I remember, It was started after installing libreoffice-help-ru package. I have reinstalled libreoffice, but this GUI show: no white background for panels, solid (2 pt) black borders are white (see screenshot).
KUbuntu 16.04, LibreOffice 5.1.6.2
E.g., elementary style: