Make sure that when you upgraded from v7.5 to v7.6 that you installed the new themes and media tags (such as youtube); simply use the import function (in the style editor and the media tag editor) and import the data in the /install/custom_codes and /install/styles folders. I think that you should just be able to add a max-width: 500px; on the .video-container class (in common.css) which should limit how large your video can be.
We overhauled just about every faucet of UBB.threads for the v7.6 line, and a good portion of these updates affect mobile capability and standardization.
