You can add your own LibO Personas or Themes as Firefox now calls them. You should be aware that Firefox have changed their support code. As a result LibO have provided their own support independent of Firefox and provided several basic Personas that can be invoked using, with Linux LibO, TOOLS > OPTIONS > Personalisation. This support is available with Version 6.2, but subject to modification.
If you are using LibO 4.3 this is still using the older support. See https://bugs.documentfoundation.org/show_bug.cgi?id=93599 and the attachment 127480 for more details on how to install your own Personas offline.