If you’ve ever tried using/making a Tumblr theme with content wider than 500 pixels, you’ve probably run into the problem with the Video-500 tag ignoring your embed code and stubbornly resizing embedded videos.
All the hard work here was done by Matthew Buchanan & Hayden Hunter. This code was included in the theme I’m using and it was designed to get rid of the letterbox around widescreen videos. Since it replaces the height of the embed code to do this, I simply added a few lines to replace the width as well.
Simply paste it somewhere in your Tumblr theme and change
contentWidth to whatever you like.