A quick run down on CSS Variables and How To Use Them. Note the var() function can take 2nd parameter for a default value.

:root {  
	--primary-color: #333;
}
div {  
	color: var(--primary-color, #444);
}
p {  
	background-color: var(--primary-color);
}
.fancy {  
	border-color: var(--primary-color);
}