Another method which is easier and does not require editing any files .. is to use jQuery to change the background color.
The following example would change the background color to yellow if using the "default" Repository template
1. Edit the Module Settings
2. Expand the Advanced Settings page
3. Enter the following into the Header field
<script type='text/javascript'>
$(document).ready(function(){
$(".RepositoryContent").css('background-color', 'yellow');
$(".RepositoryContent .normal").css('background-color', 'yellow');
$(".RepositoryContent .tmain").css('background-color', 'yellow');
$(".RepositoryContent tr").css('background-color', 'yellow');
$(".RepositoryContent .Head").css('background-color', 'yellow');
$(".RepositoryContent .SubHead").css('background-color', 'yellow');
});
</script>
4. Click Update at the bottom of the page.
That jQuery script will change the background color of all of the Repository template elements to whatever value you enter.