The only way to make videos responsive in any theme is to use javascript (
http://fitvidsjs.com/ ).
1. In your theme folder, create a folder called "js"
2. Inside /js, add the jquery.fitvids.js file
3. Inside /js, create a new file called be-video.js with the following contents:
https://gist.github.com/3868135
4. In functions.php, add this (without the <?php at the top):
https://gist.github.com/3868128