Updates the MD5 hash of the public CSS files and returns the changed ones
in a list.
@returns {Array} an array of changed files (string)
function updateChangedCssFiles() {
		let hash, filePath;
		const changedFiles = [];
		for ( filePath in publicCssFiles ) {
			hash = md5File.sync( filePath );
			if ( hash !== publicCssFiles[ filePath ] ) {
				publicCssFiles[ filePath ] = hash;
				changedFiles.push( path.basename( filePath ) );
			}
		}

		return changedFiles;
	}