The story is about module registration Joomla called Add CSS.
What can do this module?
First of all, it allows you to add CSS rules to your pages, also including an additional CSS file or alternative method of writing these rules in the module settings.
The combined settings of the destination module to a different menu or individual items can be used for individual pages, each page will vary its appearance.
When you use a class suffix of the module to change only some elements, which is inconvenient. In the case of module Add CSS, you have the opportunity to change exactly what you need.