- Go to Configure at the top menu of Admin.
- Choose Site Appearance and Assets at the left menu.
- Upload a Custom Style Attachment by clicking add.
- Proceed to customize your CSS in Stylesheet.
- On the top-left corner of the style sheet window, you will see an Image icon. Once you click on it, a selection of your saved assets will be displayed, and once you select a specific item, it will be automatically added to your stylesheet.
You can find other articles about front-end customization here.