how do I change the background color of the pages?

What app are you talking about? iBooks Author? iBooks for iOS?

Do you know how to edit the template? If not simply choose a rectangle shape, change colour to Black and using ARRANGE, send to back and bring forward till it replaces whatever is there.

The best way is to pull down the book slide to reveal the Layout - which shows you the chapters and section etc which make up a template. You have to use the same procedure on ever page from the template you intend to use.

In the Template Layout, select your page. Click on whatever your background colour is...IF its unlocked you see the square pull handles in corners - now simply select Black and it should change the colour - if not and its unlocked, it may be a black image, in which case delete it, Insert a new rectangle, pull to fit the page and change colour to black. It should now be hiding the other content, ARRANGE > Send to Back and it should sit behind all your content. Now go back to arrange and LOCK.

You should see a red marker on the left with whatever page you are editing "Save Changes" click it and its done... repeat on all other pages.

The layout has been protected, use unlock to be able to edit the page.