From a usability point of view, changing scroll bars is a no-go. The user is familiar with his/her operating system and the visual environment it provides. Changing this does create confusion, especially among those who are new to using computers, old people etc.
The majority of users today will probably recognize a scroll bar as long as it has it's basic shape - and many will not even use the bar itself, but just the scroll wheel on the mouse.
It's still a thing to think about though. Consider your audience. And whatever you do make sure it actually works on all platforms/in all browsers if you use a custom scroll bar.