How to get line under each post (<hr /> tag)

Hi

Hoping someone can help. I’d like a grey line under each post on my main blog page here:
https://www.shotsfromthesmoke.com/

Like under the ‘Latest’ heading at the top of the page

If I click a post category, eg ‘Photographs’ (https://www.shotsfromthesmoke.com/category/photographs/)the lines I want get put in automatically. I’d like the lines to show on the main page as well.

Can anyone help?

Thanks