You might want to consider a different design to add such a ribbon, it's blocking the "important" text.
The idea shouldn't be hard, in theory. You could add a column in your database table for posts labels "important", or similar. Also, it would be much easier to edit to the existing post-display and admin posting pages, instead of making something independent. More importantly, do you know how to understand and edit PHP, and your database? That would be the real hurdle in this case.