OK figured it out. I missed a part of the documentation on this page about using custom fonts with grids.
I needed to also target purecss grid classes as well as the html/body element, like so:
body,
.pure-g [class *= "pure-u"],
.pure-g-r [class *= "pure-u"] {
/* Set you're content font stack here: */
font-family: Georgia, Times, "Times New Roman", serif;
}