This means that if you do want to provide a browser fallback, it’s as easy as providing another more widely supported value immediately before your declaration.
Not all browsers support variables (https://www.w3schools.com/css/css3_variables.asp). So you need to make sure there’s a fallback for the browsers that don’t understand variables. How would you make a red background without a variable?
you just need to add
background: red;
before
background: var(–red-color);
This is only need to add because if browser can not recognize the variable i.e." background: var(–red-color);“than browser set to the main style i.e.” background: red;"