Customize Layout

I need to change the layout of the widget (meaning: the way it shows the posts on the web site) as there’s no space between the posts, so it’s hard vor the reader to check where one post ends and where the next begins.

So I simply would like to add some
‘s in the code but don’t know where.

Could anyone help me please by letting me know in which file, which function and which position I can do this?

Thanks in advance!!!

https://wordpress.org/plugins/wp-most-popular/