As one who has/had a rather large CSS file, I know that any style sheet can be reduced significantly. I am in the middle of reducing my style down to a minimum. But then again, my site is not too complicated really. It is often argued that CSS, even for an advanced site, does not have to be big.
I assume the same can be done with JavaScript. (But please do not scream at me if you disagree.)
There is a value to reducing CSS and JavaScript. But because these are text based, the download speed differences can be rather small. However, it is a good idea anyway. I am not sure I would complicate things by having dynamic CSS/JavaScript. It may be a lot of work for small gain. It may be a better exercise to just find clever ways to reduce the use of these technologies down to the smallest footprint possible then not worry about it.
BTW- I find that reducing images provides the best performance improvement overall. Something I still have yet to do.