"CSS Processor" -- "simplify" -- RIGHT.
Just.. don't. If you bother learning to use HTML and CSS correctly, you shouldn't have enough CSS to warrant the use of a pre-processor, and you'll end up writing as much, if not MORE CSS than you would probably be using without it.
They are universally a LIE -- just like frameworks. I've never seen ANYTHING done with them that couldn't have been simpler, smaller, or more useful without it, unless the design itself was "not viable for web deployment" due to being a giant middle-finger to accessibility, usability, and ux!
Admittedly, I say the same about the mind-numbing asshattery that is react... at least if you are building websites. Full stack crapplets? Different story.
But since you said a forums, don't piss on accessibility from orbit by using any of that garbage... well unless your intent is to scam the feeble minded with 'ooh, shiny'.