The theme name is showing in the post image because, I’m guessing, you haven’t uploaded a featured image for that post and it’s therefore displaying a placeholder image instead.
The post text (excerpt) is showing shortened due to a theme setting or how the theme intends it to look and nothing at all to do with your style.css file. Or, if you’ve customised any other theme files such as single.php you may have changed the_content() function to the_excerpt() which is causing the shortened text.
However, as @cjbj states, you should not make changes to the parent themes CSS files or any of the other theme files. Any customisations should be done using a child theme – https://codex.wordpress.org/Child_Themes
It might benefit you to also read the theme documentation.