I think you've got it the wrong way around. i'm pretty sure no-one actually uses IE on mac anymore. you should be more worrying about how it looks in the major mac browser safari.

by the way those new scroll buttons are worse than the previous ones. trust me, not matter how bad they look, you sersiouly have to stick to standard scrollbars. if you want people to stick around and actually read what's on your site, you don't want them spending 5min figuring out how to scroll up and down. if it takes too long for something like that they will leave.