Not cumbersome at all.

You would start by turning off all sideboxes in admin > Tools > Layout Boxes Controller, and applying the background to #contentMainWrapper in the stylesheet. You might want to keep column left/right enabled with a width setting as desired in Configuration > Layout Settings, just to keep the content clear of the background image on the sides.

The homepage content would go in define_main_page.php (in Tools > Define Pages Editor), with perhaps a wrapper div or two to apply the round-cornered box borders to in the stylesheet.

The header would get logo and background styling as required, and for starters you could enable the categories-tabs and ez-pages header bars to display essential navigation. These could later be replaced by more specialized navigation tools in the same general location.