[wp-hackers] Responsive Embedded Videos

[Funkatron]

I've run into a problem playing around with videos uploaded and embedded
into a WP post.  In A theme like Twenty Thirteen, the videos are
responsive; ie they resize when you resize the window.  In other themes,
such Highwind (https://wordpress.org/themes/highwind ), not only is the
video not responsive but it might overflow out of the content area.

I know its related to the css and I've been trying and failing to figure
out what seems to be the difference between the two themes.  I have found a
solution: the following css will make all videos responsive and max the
width to 100% the content area:

.wp-video video {
max-width: 100%;
}

Question is: is there any chance of this breaking anything?  Also if I
wanted to submit a patch, which css file do I patch it in?  there doesn't
seem to be one for media.