CSS Over-ride sub-theme
To create a new subtheme, briefly:


Create a new sub-directory in sites/all/themes with the machine name of the new theme.
Create a new file in the new subdirectory with the machine name and extention .info:



  name = The human name of the new subtheme
  
  description = ... 
  
  core = 7.x 
  
  base theme = the-machine-name-of-base-theme
  
  stylesheets[all][] = css/subtheme-new-file-name.css



Regions are not inherited and should explicitly declared again, so copy over from base file theme info file.
Color module file colors.css needs to be copied and added too, if the base theme supports it, otherwise a nasty error occurs.
Favicon is not inherited
Settings are not inherited


All the rest stay at base theme.

More about sub-theming: Creating a sub-theme