CSS to override default featured image function?

Greetings and thank you for your themes. We use catch-box theme on over 5 websites. Everything is great except one pretty problematic issue which is that it appears the featured image on posts that contain text is configured to appear as a preview slide on the posts page, automatically cropping featured image size to a default thumbnail size. For instance on the site www.bottlegourdherbs.com the front page is set to display recent posts. If you scroll down to the post from September 19, 2017 titled "Prototype Neo-Dao Cigar Box Mini Herbal Laboratory" you will see the featured image default crops the full image display. However, if a post is made with ONLY an image, also set as the featured image, the post default displays the full image (like the October 16, 2017 post). Our tech partner found that the primary fix is to create a child theme and override the default image function in content.php with the following code: Specific line: <?php the_post_thumbnail('featured-slider'); ?> to change to: <?php the_post_thumbnail('full'); ?> But is there a way to make the settings that default to an image-only post be the same as one using the featured image with that image displaying also with text in the post body? Or may you make available a Custom CSS override as an alternative fix, to make management of the many sites running catch-box more expedient and without potential problems in updates? Thank you in advance from all of our team!