Download a theme, or add this to the obsidian.css file in your vault’s folder. If there isn’t one already, create a plain text file, and save it as obsidian.css.
.markdown-source-view {
font-family: monospace
}
You can replace monospace with the name of the font you want to use. If you use monospace, it will just use your system’s default monospace font.
Make sure to have quotes surrounding your font’s name, “like this”, if the font’s name has a space in it.