Margins setup on text block

Hello,
I got an issue regarding changing the margins on a text block.
In Visual Composer, I’ve got a line with 2 columns.
In each columns I inserted a text box with a shortcode displaying a filckr gallery. Just below this element I insterted a button with a title and a link to the album on flickr.
When loading the page, I have a 40px bottom-margin on the text box element. I would like to make it a 5px bottom-margin, but only on this block…
Can’t find a way to do it in the visual composer of the page the same way I can control the margin on a headline element for exemple…
Thanks for your time!

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