Do you want to add CSS as part of a text submission written in Atto? Or do you want to change your theme?
The Classic theme uses "Boost preset files" which could be an easy way for you to make some changes to the theme behavior.
When you say that you changed the style, what do you mean?
Of course, you could modify the theme itself or make a new theme based on Classic: https://docs.moodle.org/dev/Creating_a_theme_based_on_classic
If you're going that far, you may want to post your question in the Theme forum: https://moodle.org/mod/forum/view.php?id=46