CSS preprocessor variables and CSS custom properties (often referred to as “CSS variables”) can do some of the same things, but are not the same.
Practical advice from Mike Riethmuller:
If it is alright to use static variables inside components, when should we use custom properties? Converting existing preprocessor variables to custom properties usually makes little sense. After all, the reason for custom properties is completely different. Custom properties make sense when we have CSS properties that change relative to a condition in the DOM — especially a dynamic condition such as
:focus,:hover, media queries or with JavaScript.