Your code passes for me when I add background: red to the .red-box selector.
We need to see your new code:
When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.
You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.
See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (’).
Also, make sure you do not have any dark mode/dark reader browser extensions running as they will mess with the CSS.