Is there a global command to change one font to another?

A bit late for the original poster now, but in case it helps anyone else looking for an answer to this question - this is what I did to change the font family globally in a project with a lot of style overrides.  Rather an ugly solution but it worked for me.

Change the main css file

1. Find the file

In your published project, there is a subfolder …./ assets/ css.

Find the file CPLibraryAll.css

2.  Add a css rule to assign your preferred font to a wide range of selectors

At the beginning (or after any imports statement), add the following, but obviously substituting the font-family or font stack you want to use for the one we choose, but making sure to keep the !important directive at the end:

html,body,div,p,li,span{

          
font-family: "Assistant Regular", Gotham, sans-serif !important;

}

Limitations

I don't think this would be an option if you were using a range of fonts or feature fonts - happily we weren't.

Obviously much better to get this right at the outset and work with styles as Lilybiri says.

An aside about fonts

Assistant Regular is a Google font which we linked to the project using the instructions in this helpful article:  https://medium.com/@sepyle/use-any-font-in-adobe-captivate-when-publishing-in-html5-8b03d2cabd7

One of the reasons we found ourselves changing the font is that we were concerned that Adobe may not serve their typekit fonts to our project if we don't keep up our Creative Cloud licences in future (it looked to me like this is the way the licensing works, though I may be wrong about this).