Source for file mod_switch_theme.php
Documentation is available at mod_switch_theme.php 
$change_theme =  $gBitSystem->getConfig('users_themes');  
$_template->tpl_vars['change_theme'] =  new Smarty_variable( $change_theme);  
$style =  $gBitThemes->getStyle();  
if( $change_theme ==  'y' ) {  
    if ($gBitUser->isValid() &&  $gBitSystem->getConfig('users_preferences') ==  'y') {  
        $userStyle =  $gBitUser->getPreference('theme');  
        $style =  empty($userStyle) ?  $style :  $userStyle;  
    if (isset ($_COOKIE['bit-theme'])) {  
        $style =  $_COOKIE['bit-theme'];  
    $styles =  $gBitThemes->getStyles( NULL, TRUE );  
    $stylesList =  $gBitThemes->getStyles();  
    $_template->tpl_vars['styleslist'] =  new Smarty_variable($stylesList);  
        $_template->tpl_vars['style'] =  new Smarty_variable( $style);  
 
 
        
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