The tile_back.gif is stored in the images folder of the template that your site is currently running.
As we cannot see the site, we don't know what template you are using.
The procedure requires you to make a NEW tile_back.gif image of the desired colour, and to OVERWRITE the existing image in your template's images folder.
If you don't want this background image at all, then either DELETE or COMMENT OUT the css declaration that governs it:
PHP Code:
navMainWrapper, #navSuppWrapper, #navCatTabsWrapper {
background-color:#000000;
/*background-image:url(../images/tile_back.gif);*/
color:#FFFFFF;
... and then control the colour using the HEX VALUE
background-color:#000000; (#000000 = BLACK)