Took the time to migrate to blogengine 2.9 a couple of days ago, about time since the last theme didn't even have responsive design. I really liked the default theme so I stuck with it, just made some simple tweaks to the layout.
I also made some changes to the YoutubeViewer
extension to make it responsive by linking in the following css in the main layout and changing the extension code to wrap the iframe in a div with the flex-video and widescreen classes set.
Works like a charm, iframe resizes when window size is changed to fill up the main column. Until next time, have an excellent day! ;-)