While using the theme you would want to have your own custom styling and settings, so to modify and customize the theme on your own needs you need the Custom CSS, a question some users have when using the Custom CSS is that their CSS is not being applied, sometimes you will need to add an extra rule at the end of each value – the !important rule, let’s see this example:

body {
    background: #000 !important;
}

Please also check if you have forgot any bracket { or } opened or unclosed.

Important – use valid CSS only, you may want to check your CSS for errors here (or if you are an intermediate user, validate CSS here) before saving