~/.config/fontconfig/fonts.conf

fonts.xml

<!-- Replace the comments with the fonts and save this file as ~/.config/fontconfig/fonts.conf -->
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<fontconfig>
	<alias>
		<family>serif</family>
		<prefer><family><!-- SERIF FONT HERE --></family></prefer>
	</alias>
	<alias>
		<family>sans-serif</family>
		<prefer><family><!-- SANS SERIF FONT HERE --></family></prefer>
	</alias>
	<alias>
		<family>monospace</family>
		<prefer>
			<family><!-- MONOSPACED FONT HERE --></family>
			<!-- You can specify multiple fonts too -->
			<family>Liberation Mono</family>
		</prefer>
	</alias>

	<!-- Example of changing monospaced font default size:
	<match>
		<test name="family"><string>monospace</string></test>
		<edit name="size"><double>10</double></edit>
	</match> -->

	<dir><!-- CUSTOM FONT FOLDER HERE --></dir>
</fontconfig>