It’s odd that none of the Settings > Appearance > Font adjustments you made in a new vault don’t work. Have your tried in the Sandbox vault? Have a look at the debug help.
The fonts can also be easily changed with CSS →
/*- source+live preview mode font -*/
.markdown-source-view.mod-cm6 .cm-scroller {
font-family: 'Comic Sans MS';
color: var(--color-orange);
font-size: 22px;
}
/*- reading mode font -*/
.markdown-preview-view, .cm-scroller {
font-family: 'Times New Roman';
color: var(--color-yellow);
font-size: 22px;
}
Here’s how to use CSS snippets if you haven’t before:
It also might be a good idea to check in Font Book (mac) that the fonts show up and that other apps can use them, etc.