Stacks & Blocks
Custom template
Custom templates change the look and feel of a block. Every block can have custom templates, which are custom bits of PHP code in your website's file system.
Customising a template
When you are in edit mode for a page, you can choose to set a custom template for a block. When clicking on a block, instead of hitting Edit, pick Design & Custom Template.
Click the gear icon and from the advanced menu, you will see a dropdown menu of available templates. Choose your custom template (if any available) and hit save.
In the newest version of concrete5.8 the need to open the advanced options is removed. After you pick Design & Custom Template you are given an option to choose the block template from a dropdown menu.

An example of the newest version of the concrete5.8 custom template.
