First, declare your variable in :root. This ensures that all scopes can access it.
You do it like so:
:root {
--bg-color: #0F0A0A; /* Blackish */
}
Whenever you use it, wrap it in a var() function like this:
body {
background: var(--bg-color);
}
Now it should work! I tend to forget the var() part the most.