if you reset code you will notice that in the .red-box selector there is an empty line, that empty line is for you to write the browser fallback. The browser fallback is needed in case the browser doesn’t support the use of variables, so it needs to set the color without the use of variables, like you would have done a few challenges ago before finding out about variables
the browser fallback is what the browser uses if it can’t read the variable. SO you need to create a background declaration that doesn’t uses variables so it can be used by an old browser that doesn’t support variables