to change the paddings on a per video basis, you need to add a html class to an element.
https://laytheme.com/documentation/custom-css-styling.html#create-button-style
here i show how to target a specific element.
but thumbnails inside a elementgrid or project thumbnail grid cant have different custom html classes. only the "elementgrid"/"project thumbnail grid" element itself can.
so you might want to add each thumbnail using +project thumbnail
maybe its easier if you just crop the videos using a video editing tool like handbrake