Customise Portfolio Lightbox on Wordpress Template!

I have a portfolio page as part of my website theme which displays video projects in typical thumbnail grid layout. Currently, when a user clicks on the thumbnail it opens a new page with a video player and info about the project. I would like to change how this works. When the user clicks on a video thumbnail I want the video to appear in a lightbox-type window which keeps the user on the portfolio page with the grid in the background. They can watch the video and at any time click outside the lightbox to return to the grid. Would be great if just a couple sentences of text could appear beside the video in the zooming window. This is a note from the theme developer: "What needs to be done is another option for portfolio item that will allow you to add video that will be played in lightbox. Also our portfolio list shortcode needs to be changed in order make this to load in lightbox." *No 3rd party plugins, I want this coded in. Lastly, as my theme's developers are very active I'd like to be able to keep the modifications when I update the theme. You can instruct me as to what standalone custom code or files I need to keep during a theme update. That's it. Thanks!