Correct on editing the style.css, but preferably using a CSS Editor plugin or child theme so you don’t lose changes with theme updates.
The post id is found when you look at the source code of a web page. The other method is when you are looking at all your posts in the dashboard (the admin), then mouseover each post name, at the bottom of your browser you will see the link to the post show. In that is a part that says post=123 (number will be different with each post). That is what you would use like the sample css I posted above. So if a post=123, then change the post-274 to post-123.