Sadly, still couldn't get it working. I have add the output/linkify to the news object: 

// /mod/news_custom/views/default/object/news.php
if ($full) {
    // full view
    $body = elgg_view(
        'output/longtext',
        [
            'value' => $news->description,
            'class' => 'news-post',
        ]
    );

    if (elgg_view_exists('output/linkify')) {
        $body = elgg_view('output/linkify', array(
            'value' => $body,
        ));
    }

This didn't work, so I checked on HypeWall, the scraped video links are output as attachments, so I add a same format_attachment function to it: 

$body .= formatAttachments($news);

and also in my plugin function:

// /mod/news_custom/lib/functions.php
function formatAttachments($news) {

    $attachments = array();

    if ($news->address) {
        $attachments[] = elgg_view('output/wall/url', array(
            'value' => $news->address,
        ));
    }

    $attachments[] = $news->html;

    $attachments[] = elgg_view('output/wall/attachments', array(
        'entity' => $news,
    ));

    return (count($attachments)) ? implode('', $attachments) : false;
}

but it's still not working unfortunately. can't get my head around it. 

Read the last paragraph of the readme

It works. I feel so ashamed. Thank you, Ismayil

It's included in the vendor for. I haven't updated this plugin in years

@Matt Beckett

there's no pure css way

There's CSS way ;)

Thanks for your answers.

I am working for a touristic destination and our force are wonderful landscapes. I agree with you for white backrounds, but in this special case, my site with white bg is not "sexy" enough to fit with our image...

I've managed to make a bg image, with white, transparent blocks (for activity, groups...). It looks quite good and still responsive so it appears on mobile (tested).

So if I exclude bg image, is there others ways to enhanced aalborg theme visually? To make it more attractive for the visitor?

And if I decide to continue in this way? Where must I save the pp file? and the css?

@RvR

If there is a css only way, can you give me this way?

Thanks all for taking time for a noobie like me ;)

If RvR has a css way then great!

Otherwise use my class switching method by overriding the page/default view and picking from a list of classes at random.