How to define backgrounds in Sketch Groups

Often, when you create a group, you want to set something up in the background. For example, a background color.

For example, take the following button:

A button like this looks similar to this:

To set the semantics for Kindling to know what is the background, simply add .*background. Setting it like the following will allow Kindling to format this in a way for developer code to understand that this is the background of the layout.