Libreoffice less than version 5.0 only supports GTK2 and even >= version 5.0 only partially supports GTK3, so you need to either:
1. run libreoffice < 5.0 and use a gnome theme that supports both GTK2 and GTK3
or
2. put up with some ugliness in libreoffice 5.0+ due to incomplete support for GTK3. Some GTK3 themes are worse than others with LO 5.0+ depending on what GTK3 features they use. Plain/simple ones are likely to work best. Eventually GTK3 support will be completed and it will work as expected - there is no way of knowing how long that will take.
Work is progressing on GTK3 support for libreoffice, but it's not finished yet. See this Feb 2016 blog post by the main libreoffice dev working on GTK3 support: < \- browse around his blog, he has more posts on the topic.
Phoronix forum has had several articles tracking the GTK3 work on LibreOffice. Here's a good starting point if you want to read about it: