Oh, I thought you were asking for the styles, my bad. You should be looking at the function mo_display_portfolio_content_grid_style() at line number 415 in the file framework/functions/blog-content.php for all portfolio content.
But I guess in your case since you are customising the thumbnail, you are better off looking at the file framework/functions/thumbnail-functions.php which is called by earlier file through the function call mo_thumbnail(). It’s going to be a little bit of work to understand how this generic code works but I guess you can figure it out with some effort. Thanks