Which RH version and which Layout? For RH2015, the default font is the font set in the global settings.
So sorry. I am using RH2015. Do you mean the Windows system font setting?
What you are asking about would seem to imply you are creating CHM output?
CHM output is presented using the Microsoft HTML Help viewer. That viewer uses elements of Microsoft Internet Explorer for its display engine. Perhaps Willam knows more about it, but my own guess would be that the Default font is whatever Microsoft Internet Explorer would be using.
Why are you concerned with the TOC font?
Yes, I am creating a CHM output using RH2015. Here is my concern. Years ago, we [my company] did not specify a font for the TOC, which resulted in undesirable font substitutions. For example, on one customer's computer, the TOC font ended up being an unreadable script font. This was probably back in Windows 97 days.
Since then, we always specified Verdana 8pt as the TOC font (but for some reason chose Arial as the body text font). I want to use Segoe UI Regular as my body text font and as my TOC font. RH2015 only allows Light as the font style selection when Segoe UI is selected as the font.
I want to avoid the issue we had years ago. I can specify another font for the TOC but now I am curious as to what is the source of the "default" TOC font. Changing the font in Internet Explorer is not changing my TOC font.