Confusing interface for disabling core CSS files

A checked box typically indicates that an option is enabled and an unchecked box indicates that an option is disabled (e.g., page elements like Logo, Site name, Shortcut icon, etc. are enabled by checking the box and disabled by unchecking the box).

Either way, it would be a good idea to explicitly state something to the effect that "only checked stylesheets will be loaded" or "only unchecked stylesheets will be loaded".