If you have recently installed the latest version of LibreOffice 4.2 and want to switch the icon theme from default Galaxy theme to a flat monochrome theme, then you will need to install Sifr Icon theme and enable it.
Install and Enable Sifr Icon theme
– Run the following command:
sudo apt-get install libreoffice-style-sifr
– Open LibreOffice, click on Tools > Options > View and change the icon style to Sifr