Omega theme, global css and the lack of grid at global.css
Omega is designed as a mobile-first, breakpoint responsive theme.

global.css is the mobile first sheet, so the grid system doesn't apply.  The grid kicks in with the mytheme-alpha-default.css.

The answer for What&#39;s the difference between global.css and default.css files in Omega theme? goes into detail about the stylesheet convention.  In summary:


  
  How does my site need to look on mobile?  Make global.css
  How do things change when the grid system kicks in?  Make mytheme-alpha-default.css
  How does my site look when I am viewing it in portrait mode on a tablet?  Make mytheme-alpha-narrow.css
  How do things change when I am on a normal desktop?  Make mytheme-alpha-normal.css
  How do things change when I am on a widescreen monitor?  Make mytheme-alpha-wide.css