When writing your code simply assign the "sg-" class to your box elements. That's why I don't like CSS frameworks: they throw the very idea of separation of content from presentation out of the window :(
Could you explain how you think adding a class violates the separation of content and presentation? Unless you're building a completely unstyled site, you'll be adding classes and ids anyway, so what makes using predefined ones wrong?
This makes certain types of change messier. For instance, if I have some CSS class called `quotation` that is used in various pages of my site, I can change the style of those divs or whatever in one edit of a single CSS file. However, if instead those elements all have the class name `sg-5` and I want to change them all to be `sg-7`, I must go and edit every page that contains instances of said element. And, speaking from personal experience, I will probably make a mistake or miss some cases when doing so.