The icons for a file type have nothing to do with what software you are using (unlike Windows). It is your icon theme that matters.
On Ubuntu, this means it has to be under either /usr/share/icons or ~/.icons. You will need to edit some files like /usr/share/icons/<your-icon-theme>/mimes/16/application-vnd.openxmlformats-officedocument.presentationml.presentation.svg. Here, <your-icon-theme> could very likely be Humanity.