First, elevate the file manager to Root and launch:
nautilus //:admin
Navigate to /usr/share/applications
So in the file manager, you will see usr, double click it. Then share and last, a folder named applications.
In that folder, right click on the .desktop file for libreoffice-writer.desktop
On the exec line, change it to
Exec=env SAL_FORCEDPI=150 libreoffice --writer %U
Save the file.
Test launching writer and make sure Icons are good - if so, you can close the file and the elevated nautilus File Manager.