This example checks the WPML language and uses styling settings for an RTL style instead of the regular style for your form.

add_filter( 'frm_add_form_style_class', 'add_style_class', 1 ); 
function add_style_class( $class ) { 
  $lang = apply_filters( 'wpml_current_language', null ); 
  if ( $lang == 'ar' ) { // change ar to your language code 
    remove_filter( 'frm_add_form_style_class', 'FrmStylesController::get_form_style_class', 10, 2 ); 
    $class .= 'frm_my_style_class frm_rtl'; // replace frm_my_style_class with your rtl style class 
  } 
  return $class; 
} 

This example adds a use_style param to the form shortcode. If you'd like to display the form using a different Style than the form's default Style in a particular location, set the use_style param to the name of the different Style.

Use lowercase words with dashes in between, the way the Style appears in the Style class. There's no need to include "frm_style_" at the beginning, as this is added by the code example.

add_filter( 'frm_add_form_style_class', 'frm_add_style_attribute_to_form_shortcode', 1 );
function frm_add_style_attribute_to_form_shortcode( $class ) {
	if ( ! isset( $_GET['use_style'] ) ) {
		return $class;
	}

	$new_style = $_GET['use_style'];
	$new_style = utf8_decode( urldecode( $new_style ) );
	$new_style = esc_html( $new_style );

	remove_filter( 'frm_add_form_style_class', 'FrmStylesController::get_form_style_class', 10, 2 );

	$class .= ' frm_style_' . $new_style . ' ';

	return $class;
}

Usage:

Say you've created a Busy Bee Style that you want to use for your form on your home page but not on other pages. On the home page, your form shortcode could look like this:

[formidable id=100 use_style="busy-bee"]

This code example adds a class "my_custom_class" to form ID 439. $form in this case is an array of form options that includes an array of fields.

add_filter( 'frm_add_form_style_class', 'add_style_class', 10, 3 );
function add_style_class( $class, $style, $args = array() ) {
	$target_form_id = 439; // change 439 to your form ID.
	if ( ! empty( $args['form'] ) && ! empty( $args['form']['fields'] ) ) {
		$field   = reset( $args['form']['fields'] );
		$form_id = (int) $field['form_id'];
		if ( $target_form_id === $form_id ) {
			$class .= ' my_custom_class'; // change my_custom_class to your class
		}
	}
	return $class;
}