Modify latex templates through an sphinx theme

[SebastianGode]

I'm trying to modify those table templates through a sphinx theme: https://github.com/sphinx-doc/sphinx/blob/master/sphinx/templates/latex/tabulary.tex.jinja

However the official documentation is a bit short when it comes to describing how to do that.
At the bottom of this page https://www.sphinx-doc.org/en/master/latex.html it writes the following:

Now I don't want to do this on project base, but on theme (extension) base, so that all projects have the same modifications to those templates automatically. How can I do this? Where does sphinx look for this template folder?