Couldn't create a subtheme of a core theme
In Drupal 7 if subtheme is attaching CSS with exact same file name, Drupal thinks you want to override a CSS file. That seems to be the problem for you. So you are overriding the CSS from the Seven theme by attaching style.css in the info file. 

Easiest way to get over that problem is to rename your CSS file into something else e.g. seven_sub.style.css.