When editing 'styles.css' in a course format, the styles are applied globally, they should only be applied when the format is being used. Using Moodle 2.2.3 (Build: 20120514).

As an example, attached is a course format I've developed for iMoot 2012, which admittedly has css designed to affect the whole page, but it cleanly illustrates the point as is applied on the whole site. To test, place the extracted 'imootopics' folder in '/course/format' and click on 'Notifications'. There are no database modifications. The course format appears as 'iMoot Topics' in the list of formats.

The workaround to this is a separate css file that is included by adding appropriate css sytle html tags to 'format.php'.