Hello there!
Suddenly LibreOffice looks like this, maybe it has something to do with the latest update but I am not sure. I am using v7.2.5.2 20(Build:2) and tried different settings without success. I am using Kubuntu 18.04.6 LTS (4.15.0-166-generic kernel).
Thanks in advance!
You have inadvertently switched to some tabbed GUI.
To revert to the “Standard Toolbar”, click on the rightmost button at top, looking like 3 hyphens stacked over each other and select User Interface. You can then choose your preferred GUI.
Under the traditional GUI, the same dialog is launched with View>User Interface.
Thank you for your comment. The question is that in both toolbars (I like the “Ribbon” one), panels look weird with the white background, and this was not happening before.
Maybe you have not installed the appropriate widget handling package. KUbuntu installs KDE Plasma desktop which is based on Qt (while GNOME and derivatives are based on GTK+). It is likely that widgets reverted to generic ones (X11).
Check with your package manager that libreoffice-kf5 is installed. This is the name on my Fedora machine, so it might be a bit different under KUbuntu.