Use wp_head to dinamically generate CSS
<?php
add_action( 'wp_head', 'dolly_print_styles' );
function dolly_print_styles() {
$options = get_option( 'my-options' );
?>
<style>
.my-class {
background-color:<?php echo $options['background-color']; ?>;
font-size:<?php echo $options['font-size']; ?>px;
}
</style>
<?php
}