What we're doing is configuring a responsive layout based on 960 grid system using Sass, that means that instead of applying the grid classes to our markup, we're applying them to our element IDs thus keeping a clean and semantic markup and separating content from style.
Now, enough with the geek talk and let's see how simple that is. Basically all you need to do is to set the desired values in this form:
And if you want to set different break points you may configure them on this tab:
So with this minimal effort we have set a grid based layout which is responsive and will adapt to the device it is viewed on.
As a matter of fact, even this little effort is optional. in case the default values that ship with Sasson suites your needs, all you have to do is download and enable it, done.
So why use a base theme ? because it does all this for you, because you don't want to do this again and again every time you start a project, because it keeps you up-to-date with latest technologies and sometimes it can even teach you a thing or two.
Happy sub-theming :)