code: protected InputStream resolveIntoStream(String sheet) throws IOException {
          if (baseDir != null) {
              File file = baseDir;
              for (String part : sheet.split("/")) {
                  file = new File(file, part);
              }
              if (file.exists() && file.isFile()) {
                  return new FileInputStream(file);
              } else {
                  return null;
              }
          } else {
              InputStream is = getClass().getResourceAsStream((sheet.startsWith("/") ? "" : "/") + sheet);
              if (is == null) {
                  is = getClass().getResourceAsStream((sheet.startsWith("/") ? "" : "/") + "_" + sheet);
              }
              return is;
          }
      }
docs: Resolves the given file name into an {@link InputStream}
  
  @param sheet the file to resolve (already cleaned up by replacing \ with / and appending .scss if necessary).
  @return an InputStream for the resolved data or <tt>null</tt> to indicate that the resource cannot be found
  @throws IOException in case of an error while resolving or reading the contents