plugin.php

public static function getSubscribedEvents()
{
    return [
        'onThemeInitialized' => ['onThemeInitialized', 0],
        'onTwigInitialized'  => ['onTwigInitialized', 0],
    ];
}

public function onThemeInitialized()
{

}

public function onTwigInitialized()
{
    $twig = $this->grav['twig'];

    $form_class_variables = [
        'form_outer_classes' => 'form-horizontal',
        'form_button_outer_classes' => '',
        'form_button_classes' => 'btn',
        'form_errors_classes' => '',
        'form_field_outer_classes' => 'form-group',
        'form_field_outer_label_classes' => 'col-3',
        'form_field_label_classes' => 'form-label',
        'form_field_outer_data_classes' => 'col-9',
        'form_field_data_classes' => '',
        'form_errors_classes' => '',
    ];
}