How to increase the width to the Omega sub-theme more than 950 usable pixels I have created a new sub-theme of the Omega theme (version 7.x-2.1) in Drupal 7. 

But I want to use 970 usable pixels instead of 950 pixels as the sub-theme offers in "Default container width". 

How can I do this? Shall I change the 960.css file from themes/omega/omega/css/960.css or shall I overwrite the 960.css file form my sub-theme? What you suggest?
My guess would be not to touch to the original theme and create a new one where you can override without touching anything to the original.

Now, I can't help any more, I never used that theme and Drupal 7 yet.