The files lib/gutenberg/style-editor.css and lib/gutenberg/front-end.css each contain a couple of font-family specifications that seem to be incorrect (probably due to copy/paste errors):
font-family: "Source Sans Pro", serif;
This should probably be either
font-family: "Source Sans Pro", sans-serif;
or
font-family: "Source Serif Pro", serif;
In the latter case, the Source Serif Pro font will also need to be added to line 30 of config/appearance.php.
The files
lib/gutenberg/style-editor.cssandlib/gutenberg/front-end.csseach contain a couple of font-family specifications that seem to be incorrect (probably due to copy/paste errors):font-family: "Source Sans Pro", serif;This should probably be either
font-family: "Source Sans Pro", sans-serif;or
font-family: "Source Serif Pro", serif;In the latter case, the Source Serif Pro font will also need to be added to line 30 of
config/appearance.php.