Lead Developer
Well if you have your content layout set to separate containers (Customize > Layout > Content Layout), the boxes will be separated by the background color you have set.
To place the images right up against the border, this might help: https://gist.github.com/generatepress/b252781fe296fe698b9e
Adding PHP: https://generatepress.com/knowledgebase/adding-php-functions/
You can set a manual excerpt when adding your post by using the Excerpt metabox. If you don’t see it, click “Screen Options” at the top right of the edit page, and check the checkbox next to the “Excerpt” option.