<?php
add_action( 'plugins_loaded', 'dolly_generate_css_file' );
function dolly_generate_css_file() {
    if ( isset( $_GET['dolly-styles'] ) && $_GET['dolly-styles'] === 'true' ) {
        header( 'Content-type: text/css' );

        $options = get_option( 'dolly-options' );
        ?>
            .mi-clase {
                background:<?php echo $options['background-color']; ?>;
                font-size:<?php echo $options['font-size']; ?>px;
            }
        <?php
        exit;
    }
}