CSS DOUBTS currently

I can’t understand what to do next?

So far, currently working with CSS and having a doubt about how to fix the below given challenge.

Your code so far

:root {
  --red-color: red;
.red-box {
  background: var(--red-color);
  height: 200px;
<div background="red-box" class="red-box"></div>

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36.

Challenge: Improve Compatibility with Browser Fallbacks

Link to the challenge:

You need to add a background declaration above the existing one.

You need to set the background to red inside the class .red-box