For example, I have this CSS script:
first, change the extension of your file from (e.g.) style.css to style.php . Then add this to the first line of your css:
and after that you can define the value of your background as a variable and change it easily.
background: <?php echo $value; ?>
and in your php file:
<?php value = "#fff"; ?>
<?php $value = "#fff"; ?>