You would need to edit your theme’s code to do something like this automatically, as CSS only controls styling. On wordpress.com you do not have access to those files. You would need to be using a self-hosted wordpress.org site, and were probably finding help articles for that situation on Google instead.
Perhaps there’s another wordpress.com theme available that includes the feature you’re looking for? Otherwise, a hacky method to achieve a similar goal would be to manually change the “Publish” date under Status and Visibility when you edit a post. Your theme already shows “Posted on” at the top of each post, so you could set this to whatever date you’d like.