Depending on what theme you are using, you should be able to add your own CSS in the dashboard. To do this, go to the "Themes" section in the dashboard, and select the theme that is currently active for your site. You will see a button labelled "Customize". Here, you should see an option to enter custom CSS (not all themes have this option, however).

You can also add custom CSS for each block on the front-end. When you're in edit mode, click on what block you want to add custom CSS to. When you click on it, a tooltip dialog will show, and here you will choose the "Design" option. In the dialog, select the "CSS" tab and you do your own CSS that way as well.

If you want to edit the theme's CSS files directly, they should be located under the "/packages/" directory, under whatever theme(s) you have installed).

I hope this helps?