LibreOffice is supposed to be really good, and it is really good, however on my Ubuntu systems which run KDE the interface of LibreOffice looks terrible.
I tried changing the themes and various interface settings but it does not help. Icons look horrible. The background is is horribly dark to the point of being unusable. Finally though I found a way to fix it. You have to completely remove LibreOffice and all its components, then selectively reinstall certain ones.
Use the following command to completely remove LibreOffice. You may have to do
dpkg -L |grep libreoffice to see the exact package names of LibreOffice on your system which may be slightly different.
sudo apt-get remove --purge libreoffice libreoffice-base libreoffice-calc libreoffice-writer libreoffice-impress libreoffice-en-us libreoffice-draw libreoffice-dict-fr libreoffice-dict-es libreoffice-dict-en libreoffice-ure libreoffice-debian-menus uno-libs3 ure
This will remove LibreOffice and components and libraries associated with it. Now just selectively install the LibreOffice components you want. For example to install Writer, Calc, and Impress:
sudo apt-get install libreoffice-writer libreoffice-calc libreoffice-impress
It turns out that some packages intended to “integrate” LibreOffice with Ubuntu actually mess it up. When you just install the two packages above you will get a clean interface without any of the messed-up themeing.