Yes.
In the Appearance section (where you chose your theme) of the admin dashboard there are choices including one called “editor”.
Click that link and you should see a window with your theme files files listed to the right.
One at least will be a CSS.
However, in the writing area of pages and posts you generally have the option to do visual editing or html editing. The html option allows CSS coding that overwrites the theme CSS for that page/post. I would do changes here, rather than core files.
If the theme updates, it may change the CSS file and destroy your changes there without notice.
