code: function getCustomProperty() {
    const type = NodeType.CustomPropertyType;
    const token = tokens[pos];
    const line = token.ln;
    const column = token.col;
  
    // Skip `--`
    pos += 2;
  
    const content = getIdentOrInterpolation();
  
    return newNode(type, content, line, column);
  }
docs: Get node with a custom property
  @return {Node}