Wrapping a video or image just like a post-slider

Hello all. I use a template (among others) with many layout including fullwidth – responsive. Unfortunately, contents are displayed with margins. And as you can guess, I wish I could choose whether I have an image or video embed with OR without margins.

With margins, it’s OK
Without margins, it is not OK

My idea was to simply catch the CSS style used for the post-image or the post-slider or the post-video (those without margins) and to apply this CSS into a DIV, then embed my object into that DIV.

I can play detetective to search for the proper DIV used with images, slider or videos. But I’m not able to write down the proper DIV to embed these. I’ve tried codes like

<div class="whatever_css I tried"

of course this does not work.

All the responsive or embed plugin does not work, as the result is always the same : objects are contained within the main wrap “body”, therefore, with margins.

Any idea on how to acheive this ?
Thx for the help.