youtube channel add space between thumbnails

I too would like to add some pixel space on each side and top and bottom of the thumb nails. Like Bretfrom La post 5 months ago.

Where is the style.css file and once I find it where do I put the code to add the spaces. Not being a developer, I assume the example above adds some space below the thumb nail. I also would like to add some space between the thumb nails. (Not enough for any text, just some space to show that there is different videos.

Do I do this…
.ytc_video_container {
margin-bottom: 6 px;
margin-left: 6 px;
}

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