Adding custom CSS through the theme editor might change all the sites using the theme, and there are instances that we only want to target specific site/s with the same theme on.
This is where applying custom CSS from the site editor is most helpful!
To add custom CSS through the Site Editor:
1. Pull up the site editor.
2. Click Styling from the left editing panel of the editor.
3. Click Edit Site CSS... under Local Site Styles.
4. Add the custom CSS from the available, unused lines in the CSS editor.
5. Click Save and Done buttons when finished.