If you changed the theme to a light theme for Plasma you need to adjust the GTK Theme settings aswell to a light version like Breeze for example.
LibreOffice seems to use the GTK color settings here.
Please try Settings - Gnome Application Style (GTK) - and change both GTK2 and GTK3 themes to Breeze, plus untick prefer Dark theme. This should fix it.
Thank you for answer.
I’ve set all to Breeze and all is ok, except LibreOffice.
But I can accept that, perhaps I will find another solution in the future.