permalink settings changes and affect on css page URLs

I have my site permalink settings on set on default i.e.:

mysite.com/?page_id=77

I’ve used that id format when writing my css to specify different headers for different pages, as well as for page links in my custom fields. If I change the permalink settings to “post name” i.e.:

mysite.com/sample-post/

will it break everything?

Thanks for your help.