Just put your dynamic CSS inside of a callback function, hooked into wp_print_script
, e.g.
function mytheme_custom_css() {
?>
<script type="text/css">
<?php if ( of_get_option('show_post_sidebar', true ) ) { ?>
.post{width:720px}
<?php } ?>
</script>
<?php
}
add_action( 'wp_print_scripts', 'mytheme_custom_css' );
Add as few, or as many, conditional CSS rules as you need.