In order for a JavaScript that I use to properly work, I need to add some things to the forums' CSS. But I noticed that where I did this in 3.8 (Main CSS) is no longer supported. vBulletin pointed me to StyleVars but I don't think this is going to do it (and I'm not even entirely sure on how to use it). I'm trying to integrate LiveValidation which has me add some styling stuff to the CSS. I don't understand how to do this properly via StyleVars.
If you need it today, I wrote a mod which adds a couple extra css templates to your pages. If you can wait for 4.0.1, which should be out sometime on Monday, then they will be including a new css template, additional.css, where you may add your own custom CSS to be added to your pages.