Replace thin fonts with ultralight using fontconfig

99-replace-thin.conf

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>

<fontconfig>	
	<!-- replace thin fonts for fira with ultralight, since thin is too light -->
	<match target="font">
		<test name="family" compare="contains">
			<string>Fira Sans</string>
		</test>
		<test name="weight" compare="less_eq">
			<const>thin</const>
		</test>
		<edit name="weight" mode="assign" binding="same">
			<const>ultralight</const>
		</edit>
	</match>
</fontconfig>

We can also use <int> for the weight.