Cannot apply styling to posts

Hi
I am using D5 template and have created a blog page with a post, I’ve styled the post using css, all good, however when I add any subsequent posts they appear with the default styling of the D5 template. Other than copying the css from the first post to all future posts does anyone know what I need to do in order for all future posts to pick-up my styling.
Here is the page.

Thanks.