You can change the background color, font color, font size of the back-to-top button.
How to customize the scroll back-to-top button:
1. Pull up the editor of the theme associated with your target site.
Note: If the theme is being used by other sites, you might have to duplicate the theme beforehand, and use the duplicate theme for your site, to avoid unnecessary modifications with the other sites. Refer to this article on how to duplicate a theme.
2. Navigate to Main > Site Basics > Links & Buttons > Scroll-To-Top Button or use the search bar at the top left pane and type the keyword Scroll-To-Top.
3. Select preferred options for the following parameters:
4. Click Publish button.