That is a big question. The short answer is that you need to work with XenForo's templates and CSS to modify the style to your liking. It is a process that requires styling ability (strong html / css).
Most of the HTML you want will probably be in this template:
Admin CP -> Appearance -> Templates -> PAGE_CONTAINER
That is the html skeleton used by all XenForo pages.
CSS is defined throughout the various .css templates in the same area:
Admin CP -> Appearance -> Templates
...but you will probably mostly be adding custom CSS to this template:
Admin CP -> Appearance -> Templates -> EXTRA.css