There is an official LibreOffice PPA for Ubuntu 10.10 and 10.04 (the upcoming 11.04 will have LibreOffice by default).
You need to add ppa:libreoffice/ppa in your software sources to add it to the software center, just follow the instructions for the GUI way.
Quick and dirty CLI instructions (these will remove OpenOffice.org):
Add the PPA and install LibreOffice:
sudo add-apt-repository ppa:libreoffice/ppa
sudo apt-get update
sudo apt-get install libreoffice
For GNOME integration (for people running default Ubuntu):
sudo apt-get install libreoffice-gnome
or for KDE Integration (for people running Kubuntu):
sudo apt-get install libreoffice-kde
Fabian Rodriguez adds:
If you are using Ubuntu do not download the .deb files for manual installation, there is a PPA repository that has been available for a few weeks now. Follow these instructions to install LibreOffice from the PPA so you get automatic updates.
Keep in mind a PPA is always considered a third-party application and unfit for production purposes, however LibreOffice is becoming part of Ubuntu officially in the next release, due in April 2011, so the PPA will get a lot of attention and care. Make sure you test this and perhaps wait a few weeks if you intend to use this in 10.04 LTS or 10.10 in any significant way.
Additional language modules, help files and extensions are also available if you search for libreoffice in your package manager.
[If you get an "unmet dependencies" error, you probably need to uninstall openoffice (the installation of LibreOffice will continue automatically immediately after openoffice is purged from the system)]
sudo apt-get purge "openoffice*.*"
[Also note: I got an error about Java : you may want to check that is installed before installing LibreOffice]
If you're installing via the PPA then no, it will replace OpenOffice.org with Libre Office.
If you're installing via the tarballs on their website, then tinhed's answer is correct.
Best Answer
You have the tango icon theme selected (this not a legacy Open-Office theme by the way). Also I found that LibreOffice seems to force certain themes if your GTK+ theme is changed (at least in 3.4, so they might have back ported that to 3.3.3).
What you can do is go to tools>options>view and choose another theme (such as galaxy). If you don't have that theme, then you need to install the other LibreOffice icon themes (called styles) from the Software Centre or synaptic.