Icon missing in back-to-top button

Description

When scrolling down on the website, a return to top button appears on the bottom-right corner. This button is clearly missing an image or an icon. There is a rectangular shape with letters in it instead.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://ospo-alliance.org
2. Scroll down
3. See grey button in the bottom right of the website (the button becomes orange when the cursor is on top of it)

Expected behavior

The button should display a meaningful icon (up arrow?)

Screenshots

Desktop (please complete the following information):

• OS: Ubuntu Linux 22.04
• Browser Firefox 114.0.2 / Chromium 114.0.5735.106 (Official Build)