Add custom variable to page.tpl.php using custom module?

I found this solution here:

https://www.drupal.org/forum/support/module-development-and-code-questions/2010-12-25/add-custom-variable-to-pagetplphp

I created a module for give possibility to some users to change the background file from the UI.

 

In my module:

function office_file_upload_preprocess_page(&$vars)
{
    $vars[‘background’] = “BK2.jpg”;
}

In my page.tpl.php

<?php print $background;?>

Now how can I modify permanently  after this function

file_save_upload(‘upload_item’, $validators, $location, $replace = FILE_EXISTS_REPLACE);

the above variable to change the background?

I tried variable_set() but no luck!

Drupal version: 


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