Hi
You can open the layout file that has the module position, add a div with class in the loading position. For example, I want to add background for sidebar-1 position. I will open the file: templates\t3_bs3_blank\tpls\blocks\mainbody\two-sidebar.php
You would find the following code:
<!-- SIDEBAR 1 -->
<div class="t3-sidebar t3-sidebar-1 col-xs-6 col-md-3 col-md-pull-6 <?php $this->_c($vars['sidebar1']) ?>">
<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar1']) ?>" style="T3Xhtml" />
</div>
<!-- //SIDEBAR 1 -->
Now I will add a class for the position:
<!-- SIDEBAR 1 -->
<div class="t3-sidebar t3-sidebar-1 col-xs-6 col-md-3 col-md-pull-6 <?php $this->_c($vars['sidebar1']) ?>">
<div class="sidebar-background">
<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar1']) ?>" style="T3Xhtml" />
</div>
</div>
<!-- //SIDEBAR 1 -->
Next, add style for the class "sidebar-background" with background css rule to the custom.css file.
Regards.