Why does my extended template override change when I change the main template styles?
In many cases, extended template overrides (formerly known as sub-templates) either add or override styles that belong to the main template. In these cases, styles that are not specifically set by the extended template override are inherited from the core styles of your template. If one of the core styles are changed, it will affect the extended template override. A great way to determine which styles are affecting your extended template override is by using Firebug.