For @jcatanz (and anyone else who still can’t figure this out…)
Create a new Impress file…
…and the next time you create a presentation it will open with the new default font…or any other setting (i.e. background, colors, etc.). The procedure is the same in any of the LibreOffice applications. Maybe the exact wording is slightly different but the core concept is identical.
In the 10ish years I’ve used LibreOffice (and OpenOffice before that) I’ve never created an Impress presentation in my life and, after reading @anon87010807 's answer, figured out by myself in literally 5 seconds how to do it.
I’ve demonstrated nothing that couldn’t be easily found in the Help section of every installation of LibreOffice, or on YouTube or any of the hundreds of sites on the internet dedicated to this kind of thing.
That said, as non-difficult as it is to set the default font, maybe the LO developers should consider creating some kind of direct preference setting for the LO default font since the existing method seems to confound some users.
Thank you and good night. Flag “not for the thin skinned” in 5…4…3…2…