In Concrete CMS 5.7, theme's can enable a grid layout, which lets site editors break their pages into a uniform system of columns, which can also adapt to responsive designs. The grid container can also automatically be applied around blocks to keep them within this container, allowing some of those block types to "bust out" of the container and go full width on the page. Take a look at this image
The bottom three images are in a three column layout. Notice how they're set in from the edge of the page? They are centered within the browser. The top image, however, is part of the image slider block, which disables the grid container, so it can extend the entire width of the page. Disabling the grid container is something that's easy to do at the block type level. Simply add this line of code to your block's controller.php class:
protected $btIgnorePageThemeGridFrameworkContainer = true;
Currently, this is a block-type-wide setting, but in the future it maybe available at a single block instance level.