Hi Andrew,
Well, I don´t get any error at all. It is just it doesn´t work. I write new CSS in the child-theme style sheet, but when opening the site, any diference is shown (when changing a color menu, a font…).
To check if it was a problem with the way I was creating the child theme, I have tried to do it with the Twenty Thirteen theme and it perfectly works with it.
I don´t know why, but my child theme just doesn´t overwrite the parent one. It is not that I get an error. It´s just it´s shown as if nothing was changed.