I had troubles with changing the font using OpenXml for a word document.
I first tried to google, but did not find a sutible solution. Then I tried the AI Copilot in Visual Studio, but it did give me a very good solution, then I turned to the AI ChatGpt, on my first try it gave me a couple of lines.
It just worked, here is the lines with a small alteration.
var runFonts = new RunFonts() { Ascii = “My Font” };
var runs = mainPart.Document.Descendants<Run>().ToList();
runs.ForEach(r =>
{
r.RunProperties = new RunProperties();
r.RunProperties.Append(runFonts.CloneNode(true));
});