Previewing Themes on a Page Level

We have a site design that causes us some problems when we are designing new pages.   All of the positioning is done via CSS, and the navigation bar eclipses the main drag and drop target area of the page, so when we try to add widgets, it's next to impossible to get them in there.   In 4.x, we would change the theme so that things were in line and we could easily drop things in.   I don't see a way to do that on 5.x at the page level.   

Any suggestions?

Thanks.