The grids in Frontendcore are totally configurable, you've got several columns options and you can overwrite the map $grid to personalize it:
$grid: ( container-width: 1140px, // Defines the max width of the container. Could be px or %. gutters: .50, // Defines the width of the space between columns. Should be relative. ) !global;
By default the grid is set to 98% and modifies the with and position of the columns depending on the resolution of the device. Reduce and maximize this page to see it in action.
The .grid-main class should be only added in the main grid of your page (usually called #container"). This class will add some paddings depending on the device you are.
If you want to create fixed layouts (very useful for admin panels) add the .grid-fixed class to your main grid container.