nextcloud-server/core/css/fonts.css

/* for low-res screens, use Regular font-weight instead of Light */
@media (-webkit-max-device-pixel-ratio: 1.3), (max-resolution: 124.8dpi) {
	@font-face {
		font-family: 'Open Sans';
		font-style: normal;
		font-weight: normal;
		src: local('Open Sans'), local('OpenSans'), url('../fonts/OpenSans-Regular.woff') format('woff');
	}
}

@font-face {
	font-family: 'Open Sans';
	font-style: normal;
	font-weight: 300;
	src: local('Open Sans Light'), local('OpenSans-Light'), url('../fonts/OpenSans-Light.woff') format('woff');
}

@font-face {
	font-family: 'Open Sans';
	font-style: normal;
	font-weight: 600;
	src: local('Open Sans Semibold'), local('OpenSans-Semibold'), url('../fonts/OpenSans-Semibold.woff') format('woff');
}