I’m running the Ruby v1.0.0 theme and wish to modify the content of the post’s newsletter so that it only sends the featured image of a post and its title, with the link to the main post, but not the whole content of the post.
Has anyone found a way of doing this, please?
This is not a theme question. Themes have zero impact on the newsletter format. 
(I get asked this a lot!)
If you don’t want to send the whole post, the workaround is to publish the post web-only, then send a separate (email only) post that teases it and links the full post. For bonus points, add to the emailed version a bit of javascript that redirects to the full version, so that people who ‘view in browser’ end up in the right place.