Couldn't create a subtheme of a core theme I want to do some customization on seven administation theme, I won't to copy it in my themes folder, I want to create a subtheme of it, it doesn't work for me. this is my info file

name = SEVEN SUB
description = a Seven subtheme
base theme = seven
core = 7.x
stylesheets[all][] = style.css


when I replace Seven by a theme already exists in my sites/all/themes, my theme inherits it, but for core themes it doesn't.
I tried to specify the path of seven, themes/seven and /themes/seven doesn't fix the problem.
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.