Adding class to the comment form in Wordpress

UPDATE:

Wordpress finally supports the possibility to add classes to the comments form. See Nabil Kadimi's answer to get an example.

My outdated answer:

Because Wordpress still hasn't supported this option, I've worked out the following workaround:

<?php    ob_start();    comment_form();    echo str_replace('class="comment-form"','class="comment-form your-custom-class"',ob_get_clean());?>

Now the standard class comment-form will be replaced by itself plus the custom class.

In the documentation for the comment_form() function:

WordPress 4.4.0 Introduced the 'class_form' [...] arguments.

So you would do:

// Output the comment form with a custom class:comment_form ( array( 'class_form' => 'my_custom_class' ) );

One a second thought

I prefer using hooks:

/** * Callback function for the `comment_form_defaults` filter hook * * @param Array $defaults Defaults. * @return Array          Defaults modified. */function se_8476425_modify_comment_form_defaults( $defaults ) {    $defaults[ 'class_form' ] = 'class1 class2 class3';    return $defaults;};add_filter( 'comment_form_defaults', 'se_8476425_modify_comment_form_defaults' );

This solution is more generic as you can use it to modify the default function behavior and themes you don't "own".

Since wordpress versión 4.1 (Dec, 2014) the comment_form function allows to specify a class attribute for the submit button.

Php Code:

$comments_args = array('class_submit' => 'btn btn-default');comment_form($comments_args);

Resultant HTML Button code:

<input name="submit" type="submit" id="submit" class="btn btn-default" value="Submit" />

For reference see the related ticket: https://core.trac.wordpress.org/ticket/20446