If you release a new version of your module and it needs to have a new database schema, you need to do two things to make this change work. First, be sure to update the schema structure in mymodule_schema() so that new installations of your module get the new database table structure. Second, use a hook_update_N() function,
"To add CSS or JS that should be present on all pages, modules should not implement this hook, but declare these files in their .info file."
CSS files can be added to a .info file using the following format: