Adding custom css to the entry-content container. Either margin or padding.
Also make sure if you modify core files of the theme that you use a child theme instead of the parent theme so you can do theme updates over time. If it’s just css and if you use Jetpack (plugin), you can use the Edit CSS feature instead of a child theme.
The title is outside the entry-content container, but what you could do is this:
The heading and content is wrapped in the article container. You could do it as a margin or padding. Be aware though when your site is viewed in a mobile environment, that margin or padding is going to be there.