I do not have access to Thesaurus in LibreOffice 5.3 I tried following the instructions at this AskUbuntu article but I do not have such a thing. I cannot install the openoffice link suggested below. It is not found
sudo apt-get install openoffice.org-thesaurus-en-us
I installed libreoffice using the ppa. This is what I see for my "writing aids" screenshot which is different than the other article. Clicking on get more dictionaries yields nothing.