I would like to format the color of the scrollbars that appear with an iframe in a Floatbox. Please could someone tell me where to put the CSS? Thankyou

That's not a Floatbox thing. The scrollbars are part of the browser interface, not part of the floatbox they happen to be displayed within.

A web search for "custom scrollbars" will turn up lots of information. I think the bottom line is, it's complicated, it's different for different browsers, and there are no css settings to modify scrollbar appearance because the scrollbars are not html elements on the web page.