Thanks for writing in!
I checked the website and it seems that you are using the default WordPress editor to create blog posts.
To make the changes, you can add following CSS under X > Theme Options > CSS:
width: 70% !important;
margin: 0 auto !important;
Cornerstone editor comes with built in Video player element that's comes with host of GUI controls to customize the display of Video. Once you are happy with the changes, same can be saved as Preset. Basically, Preset is like template that you can import to other parts of website. I have recorded a screencast that you can take a look wherein I have also shown Preset feature.
To learn more, please take a look at following resource.