Libreoffice offers a number of different icon sets, which can be selected from the menu Tools > Options > LibreOffice > View. The default Linux icon set is called Human, while the default Windows icon set is called Tango.
Together with Galaxy and Oxygen, they have been the available options for quite a long time (although there have been several improvements to the design of the icons).
The modern Breeze and the monochromatic Sifr (the default icon set on MacOS X) are the new additions. Sifr has been introduced with LibreOffice 4.2, while Breeze has been announced with LibreOffice 5.0.
Last, but not least, there is a specific High Contrast icon set for specific applications.
