As senior web developers we were facing with the same issues many times. There are libs available trying to provide a solution for custom scrollbars but they are providing support only for DIV elements - unfortunatelly. (Or at least we didn't find any which could offer more, even commercial stuff...)
Since w3c doesn't seem to do anything with it, and after using google (around the topic) time by time, over years without finding any reasonable progress we have decided to create and publish our own solution. Named NiceScrollbars library. It supports TEXTAREA (was not an easy issue, crossbrowser...) and multi SELECT too, not only DIVs. 1.0 release is out.
I tried to view the FAQ of this forum wether it is allowed (or how) to write anything here which is not only a possible solution but necessarily a promotion too.. ..but found nothing about it... Since this scrollbar stuff is a real pain for developers/designers I can just hope the admins will not filter out my link...
If not, you may check the library here: http://www.nicescrollbars.com.