OK, here we go.
Page A is the main host page with the IFRAME - Page B is the one in the IFRAME.
Both pages are independent of each other and B is simply viewed in a display window of A.
This means any format styles applied to the host page (A) do not extend to the page displayed in the IFRAME (B).
You need to add your style code for the scrollbars into the page that is being displyed in the IFRAME.
If Page B is the page you want to view in the IFRAME, paste a scrollbar style along the lines of the following between its <head></head> tags - don't forget that the style will only apply to the page it is on:
A different page (like the host page that has the IFRAME on it) may require alternative colours, so just code some different style tags between the <head></head> tags of it for an effect that is different to the page displayed in the IFRAME.