Use this code if you have selected "Load Formidable Styling on every page of your site" in your Formidable Global Settings, but the Formidable stylesheet still isn't getting loaded.

add_filter('get_frm_stylesheet',  'my_custom_stylesheet', 30, 2);
function my_custom_stylesheet($previous_css, $location='header'){
    global $frm_vars;

    if ( ! isset($frm_vars['css_loaded']) || ! $frm_vars['css_loaded'] ) {
        $css_file['formidable'] = admin_url('admin-ajax.php') . '?action=frmpro_css';
    }

    return $css_file;
}

Use this code example to change the folder where the CSS file is located.

add_filter('get_frm_stylesheet','frm_stylesheet_change_loc');
function frm_stylesheet_change_loc( $sheet ) {
	$sheet['formidable'] = home_url() . '/something.css'; //Replace something.css
	return $sheet;
}

//When we want to save the $css option when saving the style, we can hook in there too to generate the file.

add_action('update_option_frmpro_css','save_css_string');
function save_css_string( $css ) {
	// save the $css string in a file somewhere.
}