Thanks for the lead sir or ma’am! I found your post above after several hours going from one lead to the next (no joke—I know nothing!) lol Here’s what I did, hope it helps someone else if, like me, you find yourself in a “situation”. 
I was using the Minimal theme and wanted to substitute the font used for footers and references:
OS: Windows 10
Obsidian v0.13.23
Local vault on D:\
- Download and install your font (right-click, ~Install)
- Create new or open existing snippet CSS file
/* note the src is local and not URL,
mind the comma between my two local sources,
and no need to put the full path */
@font-face {
font-family: 'Reforma1918';
src: local("Reforma1918-BlancaItalica.ttf"),
local("Reforma 1918 Blanca Italica");
font-style: italic;
font-weight: 300;
text-rendering: optimizeLegibility;
}
/* isolate the area you need to modify, for me it was footers and references,
the key is to specify the font-family from above */
.cm-s-obsidian span.cm-footref, .cm-s-obsidian span.cm-blockid {
display: inline-block;
font-size: 0.75em;
vertical-align: text-top;
color: var(--text-faint);
margin-top: -0.15em;
font-family: Reforma1918;
font-weight: 300;
}
- Save your CSS snippet
- Make sure to reload and enable your CSS snippet if it’s brand new otherwise the changes should be automatic