Rename to #-selector - may apply to base selectors too.
Build columns mobile first. Inherit box-sizing from earlier setting - if applicable. This way the grid can be used standalone or within Persistent Grid Framework.
Remove media queries from mixins - they belong in behavior.
Comments
Rename column-padding to column-spacing - was too ambiguous.
Rename function to horizontal-rhythm.
Add quotes
Take out collections for now - they belong in a separate skin.