Trying to layer text over images/videos on mobile with CSS?

I'm trying to move the text up so it's slightly layered on top of the video. I was able to do it on desktop but I am having trouble with the mobile CSS. It'll move left or right but I can't get it to move up or down. This is the code I've been using:

@media screen and (max-width: 640px) {
  #blockidhere {
position: absolute;
margin-top: -100px;

}

Hi,

What is site url? We can check easier