Q:

Prestashop 1.6 HelperForm add class fixed-width-x1 to form selector

I am creating a Prestashops 1.6 module. After added input in HelperForm select, then in html the script added class fixed-width-x1.
$fields_form[0]['form'] = array(
                'tinymce' => true,
                'legend' => array(
                    'title' => $this->l('Add rule'),
                ),
                'submit' => array(
                    'name' => 'add',
                    'title' => $this->l('Add'),
                    'class' => 'btn btn-default pull-right'
                ),
                'input' => array(
                    array(
                        'type' => 'select',
                        'label' => $this->l('Category'),
                        'name' => 'category',
                        'required' => true 
                    ),
                )
            );

Screen Firebug
Why it works wrong?
I want add own class to select, but fixed-width-x1 override property width.

A:

This class is automatically added by the HelperForm class in the proccess of rendering the select. 
But, in my opinion, that you can do is add a class to the select and then change the style in this new class.
To add a new class to the select, only add this at the end of the options array:
 'class' => 'name-of-custom-class'

Be sure that the comma after the last option exist.
In summary, should be like this:
$fields_form[0]['form'] = array(
            'tinymce' => true,
            'legend' => array(
                'title' => $this->l('Add rule'),
            ),
            'submit' => array(
                'name' => 'add',
                'title' => $this->l('Add'),
                'class' => 'btn btn-default pull-right'
            ),
            'input' => array(
                array(
                    'type' => 'select',
                    'label' => $this->l('Category'),
                    'name' => 'category',
                    'required' => true,
                    'class' => 'name-of-custom-class'
                ),
            )
        );

Hope it helps you.
[EDIT]
The HelperForm class uses a template to make this inputs. This template is:
/[admin-folder]/themes/default/template/helpers/form/form.tpl

In this file around the line 341, you can find this code:
{elseif $input.type == 'select'}
    {if isset($input.options.query) && !$input.options.query && isset($input.empty_message)}
        {$input.empty_message}
        {$input.required = false}
        {$input.desc = null}
    {else}
        <select name="{$input.name|escape:'html':'utf-8'}"
        class="{if isset($input.class)}{$input.class|escape:'html':'utf-8'}{/if} fixed-width-xl"

In the last line you can see the place that is assigned this class with any control "fixed-width-xl".
The solution:

You can delete directly the class from the tpl, but this will affect
to all the inputs that are in the admin panel. And, in case of you want to update the prestashop version, is probably that you lost this change.
You can edit the admin css file deleting the "!important". This file is:
/[admin-folder]/themes/default/css/admin-theme.css

