In your sample website yellow line is before maincontent and after maincontent so basically content in between lines.
If this is what you wanted you can use below changes. 
Find File: \templates\gk_coffe\layouts\default.php
Find Lines: 98 to 110 which is below
- Code: Select all
-       <?php $this->loadBlock('nav'); ?>
 <div id="mainPage" class="gkWrap">
 <?php $this->messages('message-position-2'); ?>
 
 <?php $this->loadBlock('header'); ?>
 
 <?php $this->loadBlock('top'); ?>
 
 <?php $this->loadBlock('main'); ?>
 
 <?php $this->loadBlock('user'); ?>
 </div>
 
Add an extra div tag to before and after div mainPage so it looks like below.
- Code: Select all
-       <?php $this->loadBlock('nav'); ?>
 <div id="mainbackground"></div> // Your Yellow line
 <div id="mainPage" class="gkWrap">
 <?php $this->messages('message-position-2'); ?>
 
 <?php $this->loadBlock('header'); ?>
 
 <?php $this->loadBlock('top'); ?>
 
 <?php $this->loadBlock('main'); ?>
 
 <?php $this->loadBlock('user'); ?>
 </div>
 <div id="mainbackground"></div>  // Your Yellow line
 
Add following css to css/override.css and enable it in temlate settings > advanced settings > css override.
- Code: Select all
- #mainbackground { background: url('../images/yellowlinegrade.png') no-repeat; height: 25px; width: 900px; margin: auto; }
 
Change image details based on where your image is , currently for tested code image is in /templates/gk_templatename/images/
If you want to specifically load modules let me know.
See you around...