I finally figured out the reason Stylish can no longer modify certain elements such as scrollbars. It might be common knowledge, but I saw the CSS bits on nannami didn't mention it so I suppose I will share.
Between Stylish 1.4.0 and 1.4.1 a switch was made from AGENT to AUTHOR sheets by default. The main difference is that AUTHOR does not require !important in most circumstances to override the default CSS, but it cannot modify elements such as scrollbars (for what reason I do not know).
Anyways, the point is, you can specify to use an AGENT sheet (or even a USER sheet if you wanted) with /* AGENT_SHEET */.
So, to remove scrollbars in the latest Stylish, you would simply make a style with...
>---------------------------/* AGENT_SHEET */
@namespace url(
http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
scrollbar
{
visibility: collapse !important;
}
>--------------------------- 