The essay "Why variables in CSS are harmful" written by Bert Bos is worth to read which has some justified reasoning behind it.
"Adding any form of macros or additional scopes and indirections, including symbolic constants, is not just redundant, but changes CSS in ways that make it unsuitable for its intended audience. Given that there is currently no alternative to CSS, these things must not be added."
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Why “variables” in CSS are harmful ]