Some things may need adjusting; the Properties in document font, etc., if
you are bothered by it, but the snippet would look something like this for all three viewing modes (it could be condensed if you wanted the same font for all views):
/* source mode font */
.markdown-source-view.mod-cm6:not(.is-live-preview).cfont .cm-editor .cm-scroller {
font-family: 'Consolas'; }
/* live preview font */
.markdown-source-view.mod-cm6:is(.is-live-preview).cfont .cm-editor .cm-scroller {
font-family: 'Calibri'; }
/* reading view font */
.markdown-preview-view.cfont {
font-family: 'Comic Sans MS'; }
and in any note where you want the fonts changed, add cfont in the cssclasses section of the YAML/Properties. (The cssclass cfont can be mostly anything; it just seemed easier than writing custom-font 
).
---
cssclasses: cfont
---
Source mode → Live Preview → Reading view
