theme gravity forms with bootstrap

functions.php

<?php
// ADD THIS TO THE functions.php file OF YOUR (CHILD) THEME

// apply to all forms
add_filter( 'gform_field_content', function ( $field_content, $field ) {
  if ($field->type == "name") {
    $field_content =  preg_replace( '(<input)', '<input class="form-control" ', $field_content );
  } else {
    $field_content =  preg_replace( '(<input .*class=\')', '$0form-control ', $field_content );
    $field_content =  preg_replace( '(<select .*class=\')', '$0form-control ', $field_content );
    $field_content =  preg_replace( '(<textarea .*class=\')', '$0form-control ', $field_content );
  }

  return $field_content;
}, 10, 2 );

add_filter( 'gform_submit_button', function ( $button, $form ) {
  $button =  preg_replace( '(<input .*class=\')', '$0btn btn-primary form-control ', $button );
  return $button;
}, 10, 2 );