CSS can only change things visually. So, it can hide what’s already there, but it can’t make more text appear than what is actually being output to the page.
For this type of change, it’s typically best to create a child theme – which basically just means you create a new folder in /wp-content/themes/ and add a style.css file with a few comments at the top telling WP this is a child of whatever parent theme you’re using. From there, you copy only the file(s) that need changes into your child theme folder and change them there. That way you can keep the parent theme updated and it won’t overwrite your tweaks.
Themes vary in how many files they have and where they store information, but homepage code is often found in front-page.php or home.php. You’re looking for some variation of <?php the_excerpt(); ?> which you should be able to just change to <?php the_content(); ?> – though depending on the theme, you might also have to adjust some CSS to have this display appropriately.