CSS can be added to individual styles by a Page Manager, through School Website.

To work in Advanced CSS in a style:
  1. Navigate to School Website.
  2. Select Website management > Websites > (your site name).
  3. Click Styles.
  4. Locate your style and click Edit. 
  5. Select the area of your style you're working in.
  6. On the left, select CSS: Advanced.
  7. Add/Edit the CSS field.
  8. Click Save in the lower left corner.