WAIT maybe it did work? I had to restart Obsidian (not click the reload css button in settings) to get what I think is the correct solution to work.
CSS VARS ARE USED IN OBSIDIAN
:root {
--default-font: 'FiraCode Nerd Font', Menlo, 'iA Writer Mono V', 'iA Writer Duo V', Monaco, 'Courier Prime Code', monospace;
}
Replaces FiraCode Nerd Font with whatever you like, and close and open Obsidian!