Sub theme

I wanted to customize my opigno platform by creating a sub theme as explained in the tutorial present on this page tutorial: But as experienced by another user (explained in the comment), when you do create and activate a sub theme, the login on the front page breaks. Is this fixable ? How is this happening since the sub theme is supposed to override only the files listed in the directory, isn't it supposed to be exactly the same theme prior to any change? Thank you for reading. Quentin