I've had some really positive feedback from users about the new preview side panel available on all list views. I personally think it's a nice addition too (perhaps even adding the ability to edit directly from the list view in future would be great).
One thing that has been raised was the width of the panel being slightly too narrow, making to panel slightly harder to read than it need be for very wordy rich text fields.
Having a very quick look at the CSS for the side panel leads me to think using the "Branding" option is not going to be a simple work-around.
Wondered what other people views were and how you were using the preview panel.
I also think it is a good addition. Maybe later, they can add the option of 'divider' for adjusting/scrolling between the panels.
I just want to compliment the addition as well, this is very helpful.