CSS question: how & where to extend h1 post-title with additional own CSS class?

Hi,

I need to put an additional own CSS class to the h1 post-title.
I found in loop-index.php at the bottom this part of code:

    <article class="entry">
        <header class="entry-content-header">
            <h1 class='post-title entry-title'><?php _e('Nothing Found', 'avia_framework'); ?></h1>
        </header>
        <p class="entry-content" <?php avia_markup_helper(array('context' => 'entry_content')); ?>><?php _e('Sorry, no posts matched your criteria', 'avia_framework'); ?></p>
        <footer class="entry-footer"></footer>
    </article>

There I need to add my own class
.titellinie

So I changed in the code above:

<h1 class='post-title entry-title titellinie'><?php _e('Nothing Found', 'avia_framework'); ?></h1>

but when I save it and check this with Firebug, my own class does not appear there in the h1 headline:

I’m not so familiar with CSS and PHP – maybe I have made a mistake in writing the correct code, if you could please check it above.
Or do I need to edit another theme file for this post-title h1 tag?

My site is under construction, so I hope the screenshot helps explaining it.
Thanks a lot!