Welcome to Zen Cart Nancy
I think that brown thing is obtained using the image
/images/tile_back.jpg

Anyways, look in this file
includes\templates\classic\css\stylesheet_original.css
for leftboxheading etc

TR.leftboxheading, TR.rightboxheading, TR.centerboxheading, TR.singleboxheading {
background: #aaaad3;
/*background: #abbbd3;*/
/*background-image: url(../images/tile_back.jpg);*/
}

and try changing

Let us know if it works
I will take you more from there as to what to do
San