momh.php

<?php
namespace Grav\Theme;

use Grav\Common\Theme;

class Momh extends Theme
{
  public static function getSubscribedEvents()
  {
    return [
      'onTwigInitialized'     => ['onTwigInitialized', 0],
    ];
  }

  public function onTwigInitialized()
  {
  	// Thanks to Quark theme
  	// Form class mapping for Spectre CSS to apply
    $twig = $this->grav['twig'];

    $form_class_variables = [
			// 'form_outer_classes' => 'form-horizontal col-12',
			'form_button_outer_classes' => 'button-wrapper',
			'form_button_classes' => 'btn',
			'form_errors_classes' => '',
			'form_field_outer_classes' => 'form-group',
			'form_field_outer_label_classes' => 'form-label-wrapper',
			'form_field_label_classes' => 'form-label',
			// 'form_field_outer_data_classes' => 'col-9 col-sm-12',
			'form_field_input_classes' => 'form-input',
			'form_field_textarea_classes' => 'form-input',
			'form_field_select_classes' => 'form-select',
			'form_field_radio_classes' => 'form-radio',
			'form_field_checkbox_classes' => 'form-checkbox',
    ];

    $twig->twig_vars = array_merge($twig->twig_vars, $form_class_variables);

  }
}